Postage is dependant on the carrier/method used, and the size/weight
Shops may not send to the UK, but even if they don't list international postage you can email them and ask
Those that do may list very expensive methods - you can again email them and ask for a different method
Individuals in particular may not want to post internationally, tracking (if used) can go astray and buyers can easily say it wasn't recieved.
Import duty is a seperate matter, and unless it's in certain categories with differing rates (which paintball isn't) then based on the value
If the value (not postage) is below £15 then there is no import duty
(For gifts there is a £35 allowance-but note that false declarations are what attracts attention)
The duty is equal to vat, so 20%
If it's chargable then you also pay a fee to the UK carrier for handling (between £8 and £20)
So cost imports as:
$ price converted to £
Add 20%
Add $ postage
Add £20
If you contact a UK retailer then they may be able to import on your behalf. They will want a cut. But by getting a trade price and possibly doing any duty commercialy they will most likely cost you less or the same but with less hassle