In many ways accessibility, in terms of getting to the location is not really your issue to deal with. Accessibilty once there is of course a major issue for you.
Public areas need to be accessible to all, this can bring some very interesting challenges if you think about who may use the area, it's location etc. Wheel chair use is an obvious one, but even a short walk for someone with back problems can be challenging. The ability to stop and sit down at times helps a great deal. This doesn't have to 'seating' as such.
A major side issue with accessibility is encouraging people from deprived areas to want to go to/use these facilities. Here is where you are probably struggling. From talking to TMOs around the country I know there are areas where people spend most of their lives in their own community, going to the local town is a major excursion that needs planning and most people don't bother.
I don't know your experiences, and it may be that you will need to do some rethinking. Many contractors are used to getting a contract and doing it. With government bodies, trusts etc this may work on small projects but not on larger ones. Think about the way public policy is going, it is about local empowerment, involving residents/service users. What are the main issues? Education, health, capacity building - buzz word meaning giving local people the knowledge and skills to do things for themselves - and worklessness.
Do some research, what are the aims of organisation/quango, who are the top people? what are their particular hobby horses? Because these will be filtering down.
Bottom line, you need to bidding for the contract, but you will also need to be adding in something to involve local people/communities into your calculations. Possibly even to the extent of employing a community worker.
Yes it would be quicker and cheaper to just do the job yourself, but large organisations like the ones you are talking about are probably looking for more, because that is what is expected of them.
Getting local communities involved helps to give them a sense of ownership and pride, which in turn makes the project more community friendly, sustainable and used.
Lets take the forestry commission. One aim could be to encourage more families/children to use the area. Look at involving schools, toddler groups, family groups. Even think of producing a booklet that encourages children to look for particular features, and gives some facts. You don't have to produce the books, just supply them to the client in pdf format or whatever to distribute. Many secondary schools struggle to find opportunities for work experience. If the project is due to take place during this period that could be another way.
If they are wanting to encourage minority groups then approach the local mosque, or other group which these people use regularly.
Disability? get the local special schools involved, or liaise with Contact a Family.
Think about how you can use these groups, both for the project and when it is completed. On a large project you may decide to do a butterfly walk. After you are gone it will need to be maintained, a local school may be interested in taking it on long term. Or it may you will be required to maintain the project afterwards, long term local involvement might certainly help.
But what about the other end of the project, how involved are these various people in design stage? Who is making the choice between which materials to use? who is designing the rest area? Deciding what options to use? Your client has probably got a duty to consult and are looking for partners who can work with this.
Other things to consider is that various people use spaces differently. For instance there is a wealth of research on how young people use open spaces. Anti social behaviour is often inappropriate behaviour in the wrong place. Playing ball games where young people are likely to make a nuisance of themselves or break windows in anti social. Therefor you 'legitimise' their behaviour by providing areas where they can play ball without causing so much disturbance, or break windows. You will still get young people playing ball in the wrong place, but you minimise the problem. The same with open spaces. By involving local people you will get to learn their expectations and aspirations. These can in turn be fed back to the client.
You might also want to look at other resources you can tap into. For instance if you are working with a council estate you might want to link them up with Trafford Hall where they can learn skills like planning community gardens. Training is free for social tenants, and they will help with travelling costs.
In effect you are now covering all the main bases, education, health, local consultation, empowerment, capacity building, worklessness. You are getting the local communities engaged with organisation and probably helping to meet one of its core aims. You have created a huge press opportunity and advertising that would be expensive to pay for. You will probably find you are also able to tap into other funding opportunities for the project to meet some of the objectives. If you do use Trafford Hall what you won't see advertised is that you will be tapping local groups not only into a range of training opportunities but also a funding stream they can use. Trafford Hall offers grants of up to £3000 for groups who use their training.
It may take a lot of work on your part and it will probably mean a different way of working for you, but it would certainly put your bids head and shoulders above those who are simply applying for the project. I would be very surprised if you had a couple of successful projects that it didn't start opening other doors to large contracts, and also smaller community projects.
Another thing to consider, and this is just starting to catch on is participatory budgetting. On a large project you will set aside part of the area and let the users decide what will go there. For instance you may find in a largish forested area there are people who want to use it for cycling. Remember what I said about anti social behaviour? They might decide that they want a cycle path which will test them. So you put aside part of the money and you approach the cyclists and ask what they want, and work with them and your client to create an area where they can do their thing without causing a nuisance or possible accident with pedestrians. They get their say in its design and possibly location. They are happy, your client is happy other users are happy.