I'd say that for most people, a modestly priced mesh system is the answer. For a small to medium sized property and speeds up to 500 Mbps you'd likely get away with a two unit mesh (like the one I use) that you can find for around £75, for larger properties or those with difficult wifi environments you might want to spend a bit more for a similar three unit setup, at around £100. Obviously shop around, don't assume that the big A is the cheapest source.
If the starting price is £70, for a competent mesh, then the other options - in my view - fade away. Competent boosters are around £25-40 if you want dual band, but aren't anywhere near as capable as a mesh or decent router, and may not perform at all well if they rely on the mains electrical wiring setup. A capable entry level router can be had for around £45, but creates a single wifi access point - again for "only" another thirty quid you could have a competent mesh setup.
Of course, if you can make yourself qualify for the Volt offer, eg by simply transferring an existing pay monthly mobile contract to O2, then you could get hold of some wifi pods that will probably sort things out, but you need to work out whether there's a net cost to you of doing that. If there's no cost then it's a great offer, if it cost you £5 a month more than over a year you'd have spent almost the same as buying a mesh system, and stll be on the hook for that extra fiver.
But before doing any of those things, are you 100% sure this is wifi related, rather than a broadband problem? A new router, mesh, wifi pods or anything similar can sort out wifi capability issues, but if the underlying problem is flaky broadband then those will remain.
