I think you need to sell your soul to GD to get the price down.
You have to be careful when paying from the UK. OnlineNIC charges a small %age to use a credit or debit card, and some credit cards charge for an overseas transaction if you pay in dollars.
At the moment I'm using a Nationwide select credit card to prepay with Name Silo. NS don't penalise me for using a credit card, and Nationwide don't charge a transaction fee for the dollar payment. Also I get up to two months to pay and half a percent cashback from the Nationwide. I know it's only pennies, but it keeps the cost down to a competitive level. It's under £6.80, and I could probably negotiate that down if I moved 100 or so names, and I'll probably do that in the near future.
As a bonus, I get some extras free from Name Silo. - privacy for example.