I will ask the company's tax advisor if I went into it, but just wondering what the view of others are before, as I expect he will look blankly at me for a while...
My gut feeling is that exchanging GBP for BTP is selling (you give me money and I give you BTC codes), so VAT should be paid with VAT receipt. Buying BTC of UK/EU businesses would mean that have to charge VAT and provide a VAT receipt.
Any thoughts?
For MtGox we had to do something complex, as when you trade you are not exchanging with someone (you'd need to know their identity), but you exchange with us directly. Accepting funds from various places (Europe, US, and more to come) has made this problem complex, and the situation we have found is easy: when you add funds to MtGox from Europe (for example), the European company only escrows your funds as a service, and you still deal with our company in Japan. The escrow fee is collected from you (the 2% withdraw fee) and paid by Tibanne Japan to the French company on withdraw.
The same kind of setup is used for the US company.
As for Japanese users there are not many at this point, however we "offer" 5% automatically to those, and get our funds back on the other side of the trade as tax refund. That's not really practical however and can end costing money if there are more buys than sells. I am working on a solution for this.