Sheldon Brown the man himself says:
''The bottom bracket threading on Raleigh Twentys is Raleigh's proprietary 26 tpi size. The shells are also rather wider than standard, approximately 76 mm. If you wish to upgrade a Raleigh Twenty for modern cotterless cranks, you have 3 options:
1.
Have the bottom bracket shell re-threaded to take an ISO standard 24 tpi bottom bracket. This works, but the threads are not terribly strong, because the diameter is the same, so there will be some missing threads. (The shells are not thick enough to tap out to the larger Italian size.)

2.
Re-use the original 26 tpi cups with a long cotterless axle. "7" series bottom bracket axles are made to work with 73 mm "oversized" shells, and some of them can be made to work in the Raleigh cups.

3.
Use a cartridge bottom bracket with Phil Wood 26 tpi retaining rings. I'm told that it is possible to use Shimano UN72 bottom-bracket cartridges with the Phil Wood rings, if a Phil Wood bottom bracket is beyond your budget.
If you order a Phil bottom bracket for a Twenty, you should be careful to order a long-barrel bracket (made for a 71-75 mm BB shell), or tightening down the mounting rings may not be possible."

He claims re-tapping will leave you with some weak threads...