I would totally agree with Gilbertd.
I haven't converted a car in the UK yet, but it took 3 days when I did it in Germany 4 years ago, on a very simple 1.6l, 4 cyl. machine (VW motor 1F).
Generally, I would suggest to really talk to different converters/installers and let them have a look on the actual motor! Otherwise you run the risk that they just install something cheap, where they generally do not install lube systems etc.
Apart from that, you can/should check with the different LPG device manufacturers, e.g. Prins, BRC, etc., and the Borel list what system are recommended for your motor; just to get an idea what should be installed with your motor
I am currently looking in the SW London area as well and think , once I got a new car, I will visit this garage: http://www.bpvservicing.co.uk/how-to-find-us.html
It seems that they have a little, but good reputation here.. Can anyone provide further information on them?
I hope this helps!