"forgot to mention in my initial post that I currently don't satisfy the 100 hour requirement"
You mentioned it, which is why I brought the 28 day validation to your attention. That validation is only needed for PIC flying in UK-registered Part-21 aeroplanes. Dual flying training doesn't use up the days. The counter is reset at the end of the year, whereafter a fresh 28 day validation must be sought.
The validation requires the licence to be valid for the intended purpose, eg non-commercial operations in single-engine pistons. That means the medical certificate needs to be valid. There are over a dozen CASA-designated AMEs in the UK and a few more sprinked across the rest of Europe.
An ICAO-compliant pilot licence is automatically deemed valid for UK-registered non–Part-21 aeroplanes, although I don't think flight time on these is accepted towards the Part-FCL IR/CPL/ATPL experience requirements.