It's tricky - a pasty is more of a lunchtime food that you buy from a bakery, and you'll tend to find pubs around London will only be serving mediocre reheated ones at best.

I'd generally say that the best pasty around London would be from the West Cornwall Pasty Co.

(Also bear in mind that a genuine Cornish Pasty has to be made in Cornwall. Otherwise, well, it's a corny pastiche...)

There is a WCPC in Ealing Broadway station (direct train from Heathrow). While that's hardly a dinner destination, there's nothing to stop you from enjoying the pasty and moving on to a pub - there's a fair few good ones there.