I've just looked at electoral registers for another area, am assuming the dates would be the same all over the country
If it helps, the relevant dates seem to be as follows
two registers dated 1945
May 1945- qualifying date 31 Jan to be included on the register published 7th May
October 1945 -qualifying date 30 Jun to be included on the register published 15th Oct
1946 (only one register)- qualifying date 30 Jun to be included on the register published 15th Oct
1947 (only one register)- qualifying date 30 Jun to be included on the register published 15th Oct
Some libraries may be willing to look for you (perhaps with a small fee but I've had a few who, given the address and name and maybe 2 register years, will do a look up for free)
So if this is your only reason for a long trip its always worth asking, even with a small fee it would be a lot less than the travel costs.
Boo