Hi,
I wanted to second the Mascot recommendation and recommend a company like Qualitas who are basement specialists AND house refurb / extension specialists. I have used Qualitas in the past (they did our basement when they were Pure Basement) and they had a 'foreman' who liaised heavily with the architect and project managed it. I didn't ever feel the need that we should have had a separate PM (but I was around & onsite every day!).
Mascot did a quote for us when we were considering extensions & refurb and their quote included the PM side. They are both on the 'pricier' side but I also got a quote from Avian and when I broke it down I think it wouldn't have been far off in the end as I would have needed to get a PM, interior designer & there was no break down for other costs such as ironmongery, only white sockets & switches, the bare minimum & I got the feeling that the quality wouldn't be that good. Mascot (& Qualitas) broke their quote down so comprehensively that I could see how much various elements were costing.
I would also recommend having a chat to the Good London Builders:
http://www.goodlondonbuilders.com as they are PMs first & foremost but have all the contacts for the relevant tradespeople. We were really impressed with them & they have done a lot of work BTC.
In the end we didn't go ahead with our building work but if we do in the future, I would be using one of those 3 companies.