You can claim a capital allowance of 100% in the year pf purchase if you only use it for business purposes. If you use it for private photos as well, then you can claim tax relief based on the business proportion which you would need to estimate in a just and reasonable way.
Under self assessment (as its name implies), it is up to the taxpayer to initially determine his/her own tax liability, so one would just use a reasonable proportion based on your expectation of business/private use. Within the enquiry window of 12 months from the submission of the tax return, it is possible for the Revenue to query any entry on the return which could, of course, include the proportion of private use. It is the same with the private use of a car, where the best advice is to keep mileage records to show the split. So, to be really picky, and to discourage the enthusiasm of the previous posters here, one could keep a record of the business and private photos.
That's the formal answer. However, the practicalities are that very few tax returns have enquiries, so the risk of the proportion ever being questioned is minimal.
I'd love to be in the meeting where you try and persuade the HMRC inspector that you need a £1300 DSLR to take before and after pictures of landscape jobs!
id say it was relative - if your earning 25k or so , a sthat camera would last 5 plus yrs maybe put down £300 p/a is very 'fair' . a web site and maintenance could cost £300 p/a easy for business.
you would be surprised what you cant put down as business running costs.
i think a good camera is more important as a lap-top in a landscapers/ gardeners business...