I went to a Slow Food pork pie tasting yesterday. Mrs King's (Borough Market) was far and away the best.. Ginger Pig also pretty good. For non-Londoners, Perfect Pie ltd do a pretty good one with onion marmalade giving a slight exotic tinge, available by mail order.
Avoid at all costs (£2.50 for a small pie) Lune Valley pies. Truly horrid.
For further debate on this subject - see this thread from a few months ago ...
Also, on the subject of Ginger Pig - I bought one of their sausage rolls from a shop on Old St fairly recently, because I'd heard so much about them and it looked delicious. Well, I have to say that I didn't like it at all - it was way too rich for me, and I ended up giving most of it away, and felt queasy for the rest of the day. Are their pork pies the same?