PHEV is no different to any other vehicle, basically it all comes down to the terms and conditions of your lease so you either need to check the documentation or, as suggested, speak to them on the phone to clarify.
If you are already 2k miles over the scheduled service, and still have 3 weeks to go, then I think you will likely be told that you need to have it done. Although there is usually an allowance either way around the timing of a service, this tends to be 1k miles so if you hand it back and have missed a service by 3k or so miles I think they will treat that as a breach of the lease conditions.
Getting the service done yourself is likely to be cheaper than handing it back and risking them applying their own fee, penalty, or inflated figure for the cost of a service.
Apart from that so as much as you can to have it in good condition, clean and tidy, cover up as many minor defects as you can, make sure you have both keys, locking wheel nut key, service book, etc. Anything basic that is missing just gives them the opportunity to whack on a big fee.