Because its not always just about a focus on spinning out in the small cog on descents. I found when I had a compact that I'd spent most of the time in the bottom 2 or 3 gears on the rear cassette, and by moving to a semi compact I had a much better chain line and more efficient drive train for the gears I was in most of the time. I also found the jump between the two front chain rings just felt too big on a compact, and as a result I found myself cross-chaining more in the small ring too.

To the OP I would say go for it, if you can get up the hills around Bristol without too much strain on your current set up, you will be fine on a semi compact. My sister lives in Bristol and I visited her and cycled around the area, I've only got bikes with semi compact and regular 53/39 and don't have any cassettes with bigger than a 25 on (and I'm an 85kg pie eater), so you'll be fine with the 28.

And depending on your crank you may just need to only change the rings. I had a compact Campag one that allowed you to just fit new 52/36 chain rings to the same crank, sounds cheaper in theory but of course it was Campag so it was anything but, LOL, think I got compatible TA ones in the end.