My common sense (I know, I shouldn't use that in these cases) says that it's the date of the skills test that's relevant, but the regulations state 'issue of licence'. I would try contacting the CAA and see if you can get someone to explain to you how much time you should allow between the skills test and end of the validity period, so that the CAA can issue the licence in time. If they cannot quantify that, I'd say that they should issue the licence. Hope you can get this sorted!
(c)Validity period
(1) The successful completion of the theoretical knowledge examinations will be valid:
(i) for the issue of a light aircraft pilot licence, a private pilot licence, a sailplane pilot licence or a balloon pilot licence, for a period of 24 months;
(ii) for the issue of a commercial pilot licence, instrument rating (IR) or en route instrument rating (EIR), for a period of 36months;
(iii) the periods in (i) and (ii) shall be counted from the day when the pilot successfully completes the theoretical knowledge examination, in accordance with (b)(2).