With 80 hours from the UAS, you should be able to do a single flight and get an ATPL providing you covered the nav syllabus while in the UAS. Don't let anyone con you into thinking that you need to do the 45 hours again to get an ATPL - you simply need to do differences with the civvy world and some refresher training, eg radio licence (cheap), square circuits (easier), and some different joins etc.
Plus you can use all those hours that you logged towards your total time for fATPL issue as well. Furthermore, if you have been logging your UAS hours as T/O to Landing, you are entitled to add 5 mins at the start and 5 mins at the end of every sector for taxi time, (or 10% of the total, but that is I think only for former fast-jet convertees?). Also make sure that you include the running change sectors as separate sectors for the increase in the time that you are allowed to log per the CAA. This obviously arises as you are allowed to log time from when the aircraft came under you control, until the engine is switched off and brakes applied in the civilian world, but only flying time in the air is included in the RAF logging system.
Adding IMC and night seem a but of an extravagence to me for the PPL side of things. If I were you I would prove yourself by converting to the PPL, then go straight for the licences rather than doing IMC and night - if my understanding of the current system is correct, then this will all be covered in an ATPL course.