Yes you can come back in on the existing visa. You can renew up to 15 days late (after expiry) but not recommended, do it as soon as possible, whether you will get it back in time to return to the UK (if you're going back) .... ask at the passport police how long the wait might be.
Not sure on an agent as we do our own, I do know that Carole & Tayfun do this service, no doubt others will come up with more options.
Of course they may have opened the new office to do RP's under the new rules by then, but I doubt it somehow.
