With just the two of you I definitely recommend a Hymer, the B564 and B544 are the perfect layouts for a couple. Short and compact, but very spacious inside. The bed drops down over the cab and can be left made-up, but doesn't take up any of the living space during the day (like a fixed bed) or add extra height to the vehicle (like an over-cab). The post-1994 facelift models will be just within your budget.
For £15k you will get a better 'classic' German-built motorhome from the 1990s than you will a post-2010 UK built MH. Be wary of anything on a Ford Transit chassis unless it's got improved security, an alarm and a tracker - they're very desirable to thieves (who want to strip it for the engine, gearbox, etc.), and some versions have very poor security.
Don't expect many bargains, MHs are going to retain their Covid popularity for a while yet. Even if a vaccine came out tomorrow it'd take a year for UK/Europe to get back to normal and self-contained 'bubble' holidays will be desirable. Brexit is going to change people's travel plans, but don't expect a flood of vehicles so come back onto the market in the UK any time soon. A few might give up after the latest lockdown announcement, but judging by the MH FB groups many are going the SORN route instead.
Avoid anything that's too cheap, there are a lot of scams about at the moment and it's also a good time to shift anything that's a bit ropey.