What's the easiest and most affordable way for obtaining UV reflectance images?

UV imaging is neither easy or cheap. For digital photography you need three non standard things for reasonable results. As with so many things the easiest is usually the most expensive:

A camera converted so it can see UV - your full spectrum conversion SHOULD do this, but some converters think replacing the hot mirror with glass makes it full spectrum, while others know glass is fairly effective at absorbing UV & silica must be used instead.

A filter (or filters) that block visual & NIR but transmit UV, as you say the best of these are £200+. Cheapskates like me should be able to get results with a pair of carefully selected cheap er filters - this route reduces the UV range possible.

The third factor you need is a lens that transmits UV well - most don't! The best here are lenses specially made for UV imaging, but these are rare & very expensize. I went for a nikon enlarging lens that was recommended by Dr Klaus Schmitt it only transmits about 30% at best in the UV but manages to transmit a bit further than any of the other lenses I've looked at (using the UV spectrometer at work).

Despite having a full spectrum camera, a combination of filters that transmit a short band of UV very well, & this better than usual lens, I've only managed some poor images that faintly show UV features - proof of concept at best. Sometime I'll have another go with some more powerful UV lighting.

Using film rather than digital changes the challenges - film is typically very sensitive to UV and insensitive to IR, so the filter only has to block visual light - there are many cheap filters that can manage this (all leak NIR). Film throws in some other problems - the viewfinder shows nothing with the filter mounted, you can't see to correct focus (like IR lenses typically focus UV differently than visual light. then exposures are complete guesswork with no feedback till the films processed

UV induced fluorescence is very much easier, than either approach to reflected UV photography. Which is probably why so many on-line sources on UV photography seem to only cover fluorescence