Austro have just slipped their mandatory start date for the ECQB2020 syllabus to the 1st of April as well. The 'old' syllabus formally dates to c2016 but in fact was very little updated from the first JAA syllabus of around the turn of the century. The questions changed over time, though.
You might be asking two questions, I'm not sure. I'm reading
Q1. Should I go for the UK CAA or EASA licence? At a very simple level the choice of licence depends on your right to work. If you have the right to work in the UK only choose a UK licence, if you have the right to work in the EU only choose an EASA licence. It is now complicated by the option to hold both licences, The UK CAA have said that they will set up a simple process to issue a parallel UK licence to people who once held a UK Part FCL licence (which we might interpret as for instance a PPL, but not an NPPL) who gain an EASA licence in the next (probably) 2 years. EASA have not reciprocated. Thus, if you want both licences, on the sparse information currently available to us you should make an EASA State your State Of Licence Issue (SOLI), get an EASA professional licence, then convert/validate it to UK CAA. Things that might go wrong with this: 1. Timing, the current info from the UK CAA only envisages an 'up to 2 year' 'transition period'. 2. Changing advice. The CAA themselves say that the legal advice they have received has changed over time, it may change again. Their current advice is incomplete and relatively fluid.
Q2. Should I follow the existing ATPL syllabus or elect to start the ECQB 2020 syllabus before it becomes mandatory? I had high hopes for the new syllabus but the first few ECQB 2020 exams have been met with howls of pain from the candidates. If it was me I would hold off if you can and start with the old syllabus. If you go Austro you can swap over later when the exams become more stable. The UK CAA will not currently allow this, I am still arguing.