1. It's not strictly a 3/4 inlet to the radiator.
2. The radiator has tail. The part going onto the radiator is 1/2" BSP male thread, as applies to the vast majority of radiators. The part to which the valve attaches is what is known as a union connection. This is indeed threaded 3/4" BSP.
3. The new valves will come with new tails. These tails are likely to have 1/2" BSP threads to screw into the radiator, but plain 15 mm tubes (chrome plated) on the valve side for compression fitting.
4. You can get valves with union fittings and they will come with 1/2" x 3/4" tails, but the TRV valves of this type tend to be a good bit more expensive than compression fitting ones.
5. The radiator looks as if is probably an imperial size. Myson do a range of imperial radiators, but they are more expensive that metric ones.
6. You may well have to make some minor pipework modifications fitting a new radiator, both for end to distances and distance from the walls.
7. My advice would be to:
7.1 Go with a roughly equivalent metric sized radiator. Go for slightly narrower if you can and use extender(s). If not, alter the pipework.
7.2 Make sure the radiator is piped in parallel with the hot water cylinder primary coil. If it is, fit a TRV to the flow connection. (The one which heats up first). If its piped in series with the cylinder coil, don't fit a TRV and make sure both valves are always at least partially open.
7.3 Go for a type 21 (finned one side) or type 22 (two panels with fins in between). You can always turn them down if it gets too hot, but its a bit of future proofing if you eventually go the heat pump route.