This is very difficult, especially in London. I've lost count of the number of times, I've either been ripped off, had shoddy work carried out or had work left unfinished by workmen.
You are doing the right thing, asking for recommendations. One company I did use, who's work was reasonable, was a company based in Streatham, called Segresil. It's a Polish company and the owner is actually female. Only thing is, it's difficult pinning them down, as they always seem to be in Poland.
So you are not overcharged, a good price to go on for labour is about �80 to �120 per day, per workman. I just had my kitchen done and purchased all the units, built in oven, sink and tap, work surface, tiles etc myself, this all cost about �600. My builder and his mate have taken about 10 days to install it, including all the electrics, plumbing, tiling, flooring, painting and decorating. I think a reasonable amount to pay for a new kitchen, including materials, is a maximum of �2000.