It depends of your need for mobility. In general, iMacs have a good value for their price (which includes a great display), but as a desktop, you’re limited for home use. If you think you’ll need some portability, I think the third option, MacBook Air and 4K monitor, is a good choice. I have a similar setup and am very happy, because I have both a great monitor for desktop use, and a portable when I need to work on the go. If you know you’ll never need portability, I think iMac is the best option. I understand from your post that you’re not sure at this point if you will ever need the portability. Just keep in mind that if at one occasion in the future you’ll need it, you’ll be screwed with the iMac, but with your third option, you’ll be safe.
Regarding monitors, you can find decent 4K monitors in your budget, but monitors with a pixel density similar to the iMac are rare and expensive. The best option in that regard would be the UltraFine 4K, but I think that they aren’t anymore sold in UK. Still, you can find very good value in a 4K monitor around £500. I don’t know them well enough to recommend you a specific model, I’ll let more knowledgable users make recommendations in that regard.
Source: https://forums.macrumors.com/threads/help-me-decide-what-mac-to-buy-going-insane.2329780/