south coast,
i am in exactly the same situation as yourself gadding about in the Single Pilot aircraft that has been certified as two crew. You are right about all the multi crew time counting for ATPL issue.
The question I have for you is why shell out for a type rating you are never going to use. I mean, do you actually need the ATPL unfrozen. If you are still working on the Beech then you can operate as either Captain or FO on a plain CPL so the is no need for an ATPL to get command. And if you are looking to move on to another airline, then the type rating you'll get from them will count as your ATPL skills test and when you go to gatwick for type rating issue they will exchange your blue licence for a puke green one.