R20 BBs are 77mm wide. At least both of mine are.
Other Raleigh 3 speeds only have as many threads as are needed for their shallow cups. Can anyone confirm that this is also true for the R20? If so there won't be enough threads to screw in Phil rings far enough to hold the cartridge. Are you in England? If so, you *might* find someone with 26mm BB taps to cut more threads. If not you are SOL.
IMHO the ONLY way that makes sense to put a modern crank in a 26tpi BB is to have it tapped to 24tpi. Anything you do to make it work with 26tpi, like using a UN-72, will be fine till you need to replace the BB several years down the road.
In the case of a Twenty I would remove enough from the ends of the BB that it can be faced to 73 or 68mm.
When I had my Raleigh Sport re-tapped, the gent doing the work just faced 1mm off each side to give me a 68mm shell. The R20 shell is thinner, so facing off 2mm would be feasible if the cutters are sharp.