I shoot the same combo, A1 + 600GM and optionally the 1.4TC
There is only one real option to replace it with if you want to lose a bit of size and quite a bit of weight, and that is the Nikon 800PF.
All the other options are compromises in the UK type of light.
I know because I don't live that far away on mainland europe.
Slow lenses are nice for africa or florida or austalia, but not for the UK.
The 800PF is rather slow at f6.3, but it fills so much of the frame that you should be ok.
600PF or any f6.3 consumer lens is going to produce flat images most of the year where you live, it would be just tóó much of a step back from the 600GM.
I have been looking for a lighter alternative for a year, and feel, based on experience, it just isn't worth it.
So I lug the 600GM around and save up for a Nikon Z8+800PF combo.
I would add the 800PF though, not replace the Sony 600GM with it.
There are (many) moments here in the grey dull light, where f4 is really needed.
When light allows, the 800PF would be a great lighter option.