quick update in case anyone is interested. After much research I discovered that I have two options for multi-function indicators. Option 1 is to buy a module from here
https://emotion-ride.de/bmw-nachruestungen/multifuntionale-blinkleuchten/ for €310 plus shipping or €610 including 4 new BMW indicators. This would give my R1200GS the latest R1250GS indicators. I also discovered that there is a huge thread on the German GS owners website complaining that the latest 1250 indicators are not bright enough and that you can't see the indicator when the brake light is on.
Option 2 was a complete set of aftermarket indicators from Weiser
https://www.weisertechnik.co.uk/product/extreme-evo-dual-function-front-triple-function-rear-kit/ which look less trendy but are much brighter. These have a better system which switches off the left rear brake light and left front running light when indicating left, and the same for the right. £439 for a complete kit.
I have ordered a set of these.