Brick Lane Bikes are really really...

From sheldon brown:

Bottom Bracket
[INDENT]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:

[]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.)
[]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.
[*]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.

[/INDENT]

Upgrading to Cotterless Cranks

There are three ways to upgrade a Nottingham Raleigh to use modern cotterless cranks: New Spindle, Old Cups

By far the cheapest option is to keep the original 26 tpi Raleigh cups, and install a new spindle with the tapered square ends needed to fit cotterless cranks. Not all spindles will fit, unfortunately. Some of them are just a bit too thick to fit the slightly smaller inside diameter of the Raleigh cups, even though they all are made for 11 1/4" balls on each side. Sometimes you can make the thicker spindles work by using slightly smaller 15/64" balls. Japanese spindles will usually fit with the correct 1/4" size. Raleigh bottom bracket shells are wider than the standard 68 mm width of B.S.C./ISO bottom brackets, commonly anywhere from 71 mm to 76 mm. As a result, you'll need a longer than normal spindle. Japanese spindles are generally designated by a simple code consisting of a single digit followed by one or more letters.

[]If the digit is "3" the spindle is made for a typical B.S.C./ISO 68 mm shell, and won't work in your Raleigh...the adjustable cup will sink in too far, so you won't be able to use the lockring.
[]If the digit is "5" the spindle is made for in Italian-sized 70 mm shell, and may work in your Raleigh if it has a 71 mm shell.
[*]If the digit is "7" the spindle is made for an "oversized" 73 mm mountain bike bottom bracket. These will generally work in most Raleigh BBs.

Just getting a spindle to fit into the frame is a start, but you may still have problems due to the overall length of the spindle. Older cranks used longer spindles than newer cranks. Axles of the type described are mostly available in longer overall lengths, which are likely to give poor chain line with newer cranks, though they will work.

You can also handsaw the BB shell if you really need it to be smaller, but you have to be skilled and may need facing if you bodge it.