Importing from abroad looks cheaper at first, but you need to take into account international postage. This in some cases costs more then the goods themselves, some companies won't send to the uk and others may have varying postage options. Pick the cheap option with no tracking and you have no idea if the package has vanished or is taking forever.
Depending on the value of the item you may have import vat/duty to pay. There are thresholds so it may not be due, but if you buy more items to make the postage viable then this ups the package value and increases the chances of charges
If charges are due then the way it works is the importing courier handles it all, and you are sent a bill. This will also include a fee which varies depending on courier and could be just £8 or £20 irrespective of the value and the duty.
You pay then await delivery
Standard duty is import vat at 20%, so you overpay on postage, get to pay the vat you would have paid in the uk and an extra £20 on top for the privilege.
Note that the value for calculations may not be the invoiced amount, it could be reassessed.
Avoid the importing tricks that people recommend such as having it marked as a gift, a sample etc. Customs know all of this and it just flags it to them. The only impact that 'gift' has is to slightly increase the threshold and possibly save a little vat
Certain items will be a waste of money for nothing - import a cylinder and it's unusable as they are to foreign standards and only a few are marked with multiple standards - even if the same bottle is marketed in the uk it does not mean the us one is marked for both countries
If customs (or the courier) are suspicious that items may relate to a RIF then you get a vcra questionnaire - this affect the obvious mag fed guns, but has also affected some standard paintball guns, including tipmann bravos. Speedball guns have not been affected to my knowledge. If you cannot come up with a defence then the package is returned or destroyed
Ak bodykits are available in the uk but this would not prevent a customs officer deciding it is for 'manufacturing' a rif. It is in the eyes of the beholder and not a specific definition
If you need it and its not available here then go for an import, but first consider asking a shop to get it for you. They deal with us suppliers already and can bring goods in at a net cheaper price