Don't do it for a C150 Mikeo. The costs associated with changing register will be several thousand pounds. For a "cheaper" aircraft the costs are such a high percentage of the value that it is rarely worth it.
If your C150 is Belgian registered you will need an export c of a from the Belgian authorities - I don't know what that involves, or how much it costs, but it needs to be done before you bring it over here.
When you get here, you deregister the aircraft (so it cannot fly until the process is finished) and apply to the CAA for registration. You will need a full star annual performed, plus whatever little "changes" the CAA demand (starter warning light, low voltage lights, whatever), plus all ADs up to date. Add on the registration fees, the c of a fees and the costs of getting the CAA to make you up a new POH.
Typically the process takes a couple of months and, at least £2000 for a small aircraft. Often it is 3 or 4k.
Spend 20k on a nice G reg aircraft and save yourself the aggravation. Once your little Cessna is deregistered you are entirely at the mercy of the engineers and the CAA - they always make you suffer.