Sorry, as above you need the adapter plate (I recently sold my vario topbox with the adapter plate) They tend to get left on the bike when they're sold, maybe because they don't have the original plate anymore.
Here's what you need
BMW Motorrad R1200GS. / R1250GS Vario Topcase Bracket Attaches to your original tail rack, so you can attach the BMW Vario Topcase BMW R1200GS WC (13-) Vario Topcase Bracket. Easy to find just off Junction 47 of the M4 the purpose built 10,000 sqft modern facility is opposite the M&P Direct...
www.ebay.co.uk
It might be an idea to see if your local dealer has any they've removed from a used GS knocking around, failing that they usually go for about £50 used if you're prepared to wait
Good luck.