I used to run a 2.5 Elite on LPG, before I sold it. I agree with all of the above comments.
If it is to be "professionally" converted at £2200 and saves 8p/mile - that's a 25000 mile payback. I covered twice that distance so I was satisfied. Arguably, you will get some of it back when you sell the car.
One problem in MK is a lack of gas stations. I think there are two or three at present (Shell at Coffee Hall and at Paulerspury, BP at Kingston).
If you are not going to do it DIY but want to work on other engine problems yourself you will need to at least know how to dismantle the injection system. It has to be removed to work on the cam gaskets, cam belt etc. The evaporator, filter, ECU and injection system all take up room under the bonnet.
Finally, if you are not a DIYer, be prepared to get into negotiations with the LPG fitter and your regular mechanic when things go wrong. They will both try to pass off the problem to the other.
As far as I could tell over 50000 miles, there was no loss of power, and, with a good set of injectors it was a quiet run. The ECU on mine (a BRC system) had to be replaced three times under warranty because they fitted it in the scuttle panel and it drowned! And yes, unless you DIY, be prepared to spend some time tidying up the installation afterwards.
There is a local LPG fitter just south of MK. If you want to compare notes, send me a PM, I'll let you know how he works.
Good luck.