Tom - Your options in upgrading a Raleigh cottered crank to cotterless is pretty limited... finding replacement square taper spindles is nearly impossible (for a Twenty) so your options are to use a Phil Wood bb as they support the 26 tpi Raleigh standard, use a hard to find Shimano UN72 with PW retaining rings, or re-tap the bb shell to 24 tpi.

Phil Wood bottom brackets are the best but it is a pricey option.

In practice re-tapping the shell to 24 tpi works well, you need a good guided tap and in the case of Raleigh Twenty's you also need to cut down / reface the shell as they are 76 mm wide. I do not know of other 20 clones but the Phillip's Twenty will accept a 73 mm cartridge since it's bb is only a < mm off this standard.

Other Raleigh models are not so difficult to work with in this regard as replacement spindles can still be found but it that wide 76 mm shell that causes the greatest issues.