They're all central tbh, and all have their own respective characters:
re. Imperial: Not much to cater for the average student budget, but the Royal Borough has it's pros: The Museums, Kensington High Street, Brompton Road (Harrods).
Beautiful area of town; first year is the only time in life living there is (somewhat!) affordable...
LSE, in any case, is just as "central" as KCL - Aldwych is literally just around the corner.
re UCL: Bloomsbury is also a lovely area, and like KCL/LSE is only a short walk from the West End.
It's also worth comparing Hammersmith & Fulham (Imperial), Camden/Islington (UCL), and wherever studentville is for KCL/LSE.
In short, they've all got their respective locations going for them. That's not something you refuted particularly in your post, to be fair - just worth highlighting
I won't wade into 'x is better than y' debate - it's mostly subjective anyway (when the league tables get whipped out you know it's a party
)