It would seem that the Twenty did not see as wide a distribution in the US (same with other Raleigh models) as it did in other countries and the 451 equipped British versions are much rarer here and built just a little differently.

Nice thing is that any 406 equipped Twenty can be retrofitted with 451 wheels and tyres and this is the wheel size the frame was designed for... Sun makes a Cr18 in the 451 sizing and this is an excellent replacement for either wheel size.