As a merchant services provider. The best gateway options for UK that we use are Payflow Pro & Authorize.net. They allow a full integration and we are able to put in a lowered rate structure for Shopify clients. If your still looking for a payment provider we are able to do 0.40% and $0.10 per transaction for Shopify clients. Compared to options like Paypal which are 2.9% and $0.30 per transaction it's a significant reduction.
If that's something your looking for, check out www.gobizarmour.com.