You can get a day ACU membership now, its paid at the time of entering the trial, I think its £3, I expect Richmond offer it, most clubs do now. The Richmond trial is a club one, their day membership is £2 with small wheel entry at £8. Again, a really friendly bunch & lots of other parents around in the small wheels to help catch etc.
You may be able to contact the secretary to check on the ACU day membership: chriswallis@richmondmotorclub.com