Building certainly let's you control all aspects but you just can't compete on price unless you want something exotic like a split ergo or fancy metal chassis.
Maybe this is what has thrown my research in shambles. I thought 70% was basically a laptop like layout (basically normal keyboard + arrow keys cramped under the right shift key and maybe a few extra keys). Maybe I'm mistaken.
In terms of I've been thinking up to £150 or the equivalent. Split keyboard optional, though, I've never really used one, so maybe it's worth it if it falls within the budget for ergonomics.
£150 is a bit short is you want something made of metal. £200-£250 would be easier.
But really, if you can afford it this is something that is likely to outlast your computer by a long shot and be use for hours every day.
There are diminishing returns at some point, but you're going to reach that point before ~ £500.
What is your budget? If you can spare a few weekends I think it is worth it, but I usually enjoy building things so I might be somewhat biased...
70% is not a thing AFAIK, there are 60% 65%, 75% and TKL.