You can ride on the French plates for one year as long as it is Registered, taxed and insured in france but allowing UK use.
You will need the French registration document, hopfully showing you as the owner for a long time and the original registration date of the bike----When you bring it in on the ferry GO TO THE UK CUSTOMS and declare you are bringing it in to stay, if you can with a valuation from a french dealer as low as you can safely get, and pay whatever "tax" they ask for ----- DO NOT LOOSE the receipt they give you as you will need it to get UK registration and they are a #$%*, plus time consumiong, to get later. The rest is relatively easy and I , for one, plus probaly many others will happily walk you through it.