How to get my full pension (AHV, BVG) as a Swiss citizen living abroad?

I am a Swiss citizen living and working in the UK after working in Singapore. I want to move back to Switzerland in some years and wonder what I have to do to still get my full pension there when I retire. Will my pension get smaller when working abroad because of a gap in my AHV and BVG contributions or can the pension I pay in the UK (not in Singapore)be transferred and fill this ‘gap’? Switzerland seems to have a social security agreement with the UK but not Singapore – will this have an impact?