Digital still cameras do not attract excise duty when imported into the UK - believe me, I've checked and they are zero rated. They do, however, attract VAT at the standard 17.5%. I believe film cameras do attract excise duty as well as VAT, but I'm not sure of the rate of excise duty as it varies from one group of items to another.
Just a friendly warning if you buy something abroad and then just stroll through the "nothing to declare" channel... I've been stopped by customs before, and they've checked serial numbers etc before letting me go. Be aware that for most expensive items (and generally SLR cameras fall into this category) UK customs are aware of the serial batches that have been properly imported into the UK - the importers provide this information to HM customs. They can therefore tell whether you've bought a camera in the UK or not, if they have a mind to check. Also, you could get snagged by customs on not only the first time you bring that camera back into the UK - you could get snagged by customs at a later date too. If you've brought a camera into the UK, declared it and properly paid VAT and duty on it, they will keep a record and can check back - if you walk through even sometime later, get stopped and claim you've had it a while and you paid duty the first time you brought it into the country, they are able to check the truth or otherwise of what you are saying.
There are old pilots and there are bold pilots, but there are no old bold pilots