Having sat down with a spreadsheet it looks like I am currently marginally on the downside. Year end Sept:

Current TP = 2090
TP for flights booked now to 1st April = 1960
TP for flights booked April to June = 1200 (first time GGL achieved)
TP for Economy flights booked June - Sept = 80
TP for flights booked Sept onwards = 1240

So for me the overlap is 1280 TP. Already checked yesterday, the costs for moving the existing Feb-Mar bookings to later in the year are not viable.
If BA had decided to set the date as 1st March I'd be massively on the upside, such is life.

The end result, I need to book another 480 TP of flights to maintain GGL through to April 26 whereas I was looking at needing 1760 TP of flights to maintain it though to October 26.

However, if I flip my June-Sept flights to Business I end up on the upside with 1840 TP in the overlap period and GGL maintained without any further trips. In fact adding one further trip before September would trigger another GUF/Joker in my current year as I'd be over 6000 TP.... Guess I'll be doing some upgrades today

That feels far more complicated than it should have been