+1 for BASin. This is what I'm using for my entry. Easy to set up, and an easier working environment that a physical Spectrum, but still well within the rules. Though I'll drag out a real one to test the final results.
FWIW, I hadn't touched Sinclair BASIC in 30+ years, but it's pretty easy to pick up again. It is after all, BASIC!
Initial tests suggest that my game is going to have to be even more simple than I expected, such are the limitations on BASIC. Perhaps a fast action game wasn't the best choice... Never mind, I'll justify the slowness somehow in the back-story. :-)