Pay the money. It will be cheaper than having a custom part created, and then they bear the burden of warranty support if the piece leaks or fails. BSP is a very odd pipe threading in the plumbing world in America, and you won't find that anywhere to speak of, except at a very specialty industrial supply house. You can't use a "regular' pipe fitting. The threads are different. There are darn few retailers around that will even be able to order something like that.

If it is just a compression fitting at 15mm, can't I use a 1/2" pipe that has been sanded a bit at the top to make the fit?

BSP threads are completely different spacing. It won't seat. Try Pegasus Racing Supplies for conversion fittings. They might have something that could help you. If not, buy the kit. This isn't something to be mucking about with experimental kludges on. Unless your homeowners for sure covers floods caused by homeowner error DIYing plumbing. Many these days are very tricky when it comes to flood coverage.

The thread angle and number of threads per inch are different for BSP vs NPT. While it can seem to work, it WILL leak.