The bombs used on the Varsity were indeed the 25lb Smoke and Flash. There were, IIRC, two tail types. One had a simple cruciform shape which was prone to damage, remember they were offloaded if not previously dropped. The other had I think 6 blades inside a cylindrical sleeve. If you write to OC RAF Pembrey Sands he should be able to help with size etc and provide photographs.
The 25lb PB was pretty similar in appearance to the US BDU-33 which weighs about 31lb. If you wanted a close look-a-like then an approach to RAF Lakenheath might be helpful. There are any number at the RAF bombing ranges but more are blown open for safety reasons and the tails are always bent on impact.