How about 1+50 at box speed for starters. More grain, then sure push a stop but I have yet to find any pleasing characteristics whatsoever with pushing in Rodinal. PMK pyro is another, and I think is much better for pushing; it's a concentrated A + B mix that has loooong shelf life and wide range of applications. Really unique grain too, when you want it.
I got mine far more grainy than I wanted by overagitating in D76 at 1:1. I was doing 5 inversions every 30 seconds, which is the usual for Kodak films but not Ilford. Even rated at box speed of ISO 400, printed at 8x10 it had a lot of grain.
What Jason wrote. Use Rodinal and push 1 for more, push two for even more. The extra contrast makes the grain more apparent. Don't like that idea? Use any developer you like and push 1 or push 2. Don't want the extra contrast? Frame loosely in camera and crop agressively when you print. Simply put, those are the things you can do. Works with any B&W film. Everything else are just variations on those themes.
As suggested - Rodinal at 1:25; Shoot with a 20mm lens, crop out the center and enlarge the $@!#$$#% out of it to get the 50mm image you wanted; shoot TMZ or Delta 3200 instead of HP; overexpose/underdevelop and then use a high-contrast paper grade...
Overexpose (three stops or more). You can develop normally, or overdevelop and/or develop in a harsh developer. Bleach back if necessary to print. I do this all the time to force the grain out of HP5 or Delta 3200.
As a starting point for normal contrast scenes for printing on a LPL diffusion enlarger: HP5+ box speed, Rodinal 1+50, 18min@20C, agitate for 30 seconds and then for 10 seconds every 3 minutes.
Gives me sharp negatives with accentuated grain. You may encounter bromide drag.
Overexpose (three stops or more). You can develop normally, or overdevelop and/or develop in a harsh developer. Bleach back if necessary to print. I do this all the time to force the grain out of HP5 or Delta 3200.
I'm just waiting to get moved into my new home before processing those rolls of HP5 I've got stored up so that they come out as grainy as I can make them. If you could do my trials for me and let me know which works best I'd really appreciate it. Or if you can wait a couple of months I'll let you know! I have my eye on Dektol and Rodinal as the likely candidates.