Why would you want to do that? It's not better.
It'd be like replacing every iPod out there with a Zune. OK, well, that was pointless. How did that help anyone?
The reason you THINK maglev is better is that existing UK rail beds have deficiencies in curves, track speed and loading gage.
Track speed is simply a matter of maintenance. Up the maintenance cycle on the existing track bed, and you can go any speed you like up to 300mph/500kph. Since you'd have to relay maglev track from scratch, it would of course be brand new, so it would seem better, but you can get there with regular rail too. The regular rail speed record is within 4 mph of the maglev speed record.
The curves you have to straighten the hard way, by buying the right land and regrading. You'll need to do that even if you switch to maglev.
The loading gage (height and weight allowable) is the biggest problem in the UK rail net, and not practicable to fix, and again, you have that same problem with maglev unless you build entirely new RoW.
Now, a standard railroad bogie is a very simple thing. Nothing powered on it. A maglev bogie is a very complicated beast full of expensive electronics. So now you need 2 types of rail, unless you actually plan to build maglev coal wagons at £3 million each.
With that kind of imaginary tera-quid, the best thing UK could do is fix the above issues with the regular railroad, then run USA-style stock at French speeds. I.E. TGV Superliners. That includes freight stock, as UK freight rail needs a serious fix-up.