Paypal actually moved to a similar system for both UK and US. So new clients will get the User, Password, and Signature. Older accounts might still have the Partner, Merchant information, but I'm pretty sure you can get the new account credentials to match the US. (actually US has those credentials if you use an existing merchant account).
Anyway I'm pretty sure you can get the new credentials even for an older UK paypal account, but if not we do still support making that update with eCart even though it wouldn't be supported directly through the powerstore admin.