Over the years I've had a Tippmann A5 and 3 Spyders come through from the US so far and all but the last Spyder to arrive incurred fees. It wasn't much as others are paying though but that might just be because they aren't valued at around the $1000 mark. The A5 incurred the largest fee which I think was just under £50. Not bad considering I had the whole 'sniper' package bullsh...
If you can avoid buying from abroad then I recommend buying within the UK. You'll pay more but you'll get your stuff invariably quicker and you'll be covered by UK retail laws. If, like myself, you have little or no aftermaket support for your particular marker(s) in the UK then you simply have no choice but to suck it up and pay whatever fees you get hit with when you buy your parts 'n stuff.