gliding does not count. the AEF does not count.
The 20hrs in the 152 can be copied into the new log book. You do not need a signature on the entries, all you need is at the end of the course the CFI/Examiner to certify the entries are correct and you have met the requirements for the training. So you will probably need to show him/her the old log book at the time if required.
The only time you need a signature is on the 2 year revalidation log book and licence and on successful completion of a skills test.
However...... Personally I would not bother copying them, I would just add the numbers up and start off your new log book with those (20hrs) and log your new stuff by adding to them. There is a section at the front of a look book that records flying experiance previous to the new logbook. Fill that in. I am on my 5th logbook now and just carry the numbers forward.
Any logbook you write these hours into initially is a part of your history and even if it is a bit tatty it is still a memento. I would not part with my RAF logbooks nor gliding for anything.