Most Raleigh Twenty bikes had ISO 451 diameter wheels. Despite the 20 name, this is closer to 21" than the common 406 BMX size wheels. So, is there a decent selection of modern tyres to fit the Raleigh Twenty wheel (451 diameter)? I've been looking out for them and here is a list (if you know of further ones, please post a comment with details - thanks!):
KENDA- Kompact (hard packed race)
- K-West (road, commute)
- Small block eight (photo left: hard packed race and trail, well regarded by MTB and cyclocross riders)
MAXXIS
- Drop the Hammer (hard packed race)
- Holy Roller (pavement and hard packed race, staggered block pattern, see photo below)

PANARACER - Minits Lite (fast road, commute, baldies pictured at the end of this post)
PRIMO
- Comet (fast road, commute)
- Champ (file tread)
RALEIGH
- Record (traditional)
- Shopper (traditional)
SCHWALBE
- HS110 (traditional style)
- HS371 Mow Joe (knobbly, photo below)
- HS377 Road Cruiser (heavy touring, 457, not 451, but would probably work)
- HS399 Durano (high mileage road)

SHYH HWA - Traditional style 37-451 (SJS Cycles ebay shop)
TIOGA
- Powerblock (bmx racing)
Most of these are either 28-451 or 37-451. You can get full details from the manufacturer's websites. As to retailers, you would have to search around, for example: Chainreactioncycles, SJS cycles, Winstanleys, Dereks Cycles on ebay, West country recumbents and Mailorderbikes, to name just a few.
Rejoice in your rubber!
Chuck