I just used the BMW configure tool to spec an M135i to the same sort of level as the GTI I ordered. Comes to £38k though clearly this is without discounts (and its better performance). GTI is around £30k. That is a fair gap, but I am surprised the 1 Series didn't end up at £40k which is sort of what I expected it to be at.
It looks like a really nice car, although personally I don't like BMW dash styling. £8k extra means its not really competing in the same way. You could swap your high spec GTI for a stock M135i but I'm unsure how many people willing to splash out on all the GTI options would then be willing to forgo them just to grab the better performance.