I'd just double check on the SEP, I seem to recall that once your SEP has been invalid for more than 2 years, there a few extra hoops to jump through to revalidate; It'll be in the CAA CAPs somewhere what the specific's are.
My SEP is out of date by a few months and I'm just waiting for the wx to clear up so I can revalidate. I'm also studying my ATPLs, all I'd say is you might considering revalidating now and doing a few hours here and there whilst you study. It will also keep you sane and motivated. Plus you can jump straight into the hour building as soon as the ATPLs are complete.
Another thing you might need to consider. If you are going to have a big gap in your flying, you will probably need an experienced instructor to help you get the rust off once you start hour building and ensure that you are up to a decent standard before commencing CPL. You don't want to start the CPL with poor flying skills, as the extra hours required at that stage to get you up to CPL standard will be very costly!