Many multinational stores are region specific. I can't buy the same model cars at a Ford dealership in England that I can here in the USA and I suspect it highly unlikely I could finance a car purchase in England through the same brand's finance office here in the USA. If you used your certificate in England, iTunes USA would have to go through the corporate accounting paperwork in both countries, the governmental and excise currency transfer paperwork, and interational currency exchange routine to transfer the funds to Apple UK. Somebody somewhere has to pay for all that, and if it turns out not to be profitable a company won't go for it.
Technically you are receiving funds from outside the UK so HM Customs might also want to get involved. Yes, I know it is a small amount in this case, but if somebody gifted you $200 in iTunes certificates then that would probably go over a daily allowance and Apple would have to hold on permitting the funds to be deposited to your account until you filled out the proper paperwork with UK customs. That's an extreme example but probably something Apple would have to factor in if they were to permit international gifting and it gets to be a nightmare.
