If you have a UK issued EASA ATPL(A), the SEP(A) rating may well be printed on the back page of the licence as "previously held ratings".
If it's there, it's an easy job of presenting yourself to an ATO for training as required (there is no legal minimum training required, although there are recommendations based on how long the rating has lapsed), followed by an LPC, which usually takes about an hour, with an examiner fee of around £130-£150 depending on where you go.
You will then need to submit the LPC paperwork to the CAA along with form SRG1119B for renewal, unfortunately along with a fee of around £90.
If the rating is not listed on the back of your licence, you may be able to find some proof of having previously had in and submit that to support your claim. Otherwise, it will be more training at the ATO followed by a skills test for the issue of the rating, which is a bit more involved than an LPC.
As for the medical, one option if you only intend flying for fun is to downgrade your licence to an LAPL if all you are able to obtain is an LAPL medical depending on the restrictions you have. An EASA PPL(A) currently requires a class 2 medical, although ORS4 1260 gives an exemption to this, you need to have the rating current, and in order to do that you may need the class 2 medical for the renewal paperwork to go through
A final option is to look into getting an NPPL(A) on the basis of your current licence. Again, would require some training and a test, and is only guaranteed for use on EASA aircraft until April 2019, but it's an option if it's the only medical you can get.
Looking at Part-Med, the wording to go with your restriction is
OML Valid only as or with qualified co-pilot This applies to crew members who do not meet the medical requirements for single crew operations, but are fit for multi-crew operations. Applicable to class 1 medical certificates only.
So, although you are limited to no single pilot commercial ops, you may be fine with getting a class 2 medical. I'm not an AME, but it could simply be they apply this restriction to all pilots over 60, as over that age you cannot do single pilot commercial ops anyway.