they have an IP range which used to be registered in the US and can cause VPN issues if you are unlucky enough to get an IP from that range
It still is registered in the US, "BT America" with a Californian address. I don't think ARIN assigned addresses are allowed to have a UK address, otherwise they would have done it by now.
You do find some ARIN blocks with UK addresses when the service is based in that RIR region. You can also find the opposite which is what they're doing with some of the old Infonet/BTA blocks.
There is an increasing number of blocks that were assigned by ARIN moving to the RIPE service region (and others), although you have to wonder if that is because of the fixed fees RIPE now have instead of paying $4,000 a year to ARIN for a /16.
Now that v4 is exhausted, the RIRs don't seem to care as much with the plentiful supply of v6.