Many thanks for @platypus and @yakut for advice.
Approach Ive taken is taking an evening train to Paris, overnighting there and then catching trains through to Ljubljana. As @Giofox suggests, this is all a bit of a PITA and more expensive than taking a direct flight but Im trying to cut down flights so this is where Ive landed
Trip for me has seen bookings on three platforms, Eurostar for the London - Paris leg, SNCF for the Paris - Karlsruhe leg and OBB for the Karlsruhe Ljubljana leg. Theres also the booking of bike on the Eurostar via Eurodespatch
The OBB booking platform suggests you can take bikes on the night train but an email sent t0 the Slovenian rail service suggests that you cant so i went with the safe option.