PB chasing over 10k and HM this Summer in the South East? That rings a bell!
Looking at my racing calendar (it's full of ifs, maybes and definites, so I'm not doing all of these...)
18th July - Brighton Phoenix 10k. Very flat. Pretty sure a handful of us will be travelling down on a Wednesday evening for this one.
15th August - London Pride 10k, Victoria Park. Flat. Really well organised race, I've done this (and PB'd) a couple of times.
29th September - Orange Apeal 10k, Hyde Park. I'm sure there are plenty of others in Hyde Park over the summer but this is one I've earmarked as a tune-up for my October HM...
7th October - Southend 10k. The course has changed slightly since I set my current PB here but it's still supposed to be flat and fast, out and back along the front.
There are a few fast Autumn HMs dotted around if you're prepared to go a little outside London. I'm doing the Great Eastern Run in Peterborough, 14th October. Flat and fast as they come.
Also if I hadn't already secured my VLM champs place (yay!) I might have looked at the Gosport HM as a back-up, 18th November. Pancake flat, possibly susceptible to coastal wind.