Simplest and easiest option would be to try a Virgin Media wifi "pod". Unless you're on the Ultimate Oomph package, or have a Hub 4 then you'll have to rent one at £5 a month, but it's a good bet if you don't want to invest in your own wifi or mesh system.
Best value would be to buy a £30-50 TP-Link Archer router for the upstairs, run an ethernet cable between the hub and the new router (always easier said than done), configure the upstairs router in access point mode*, and then live with two wifi networks to which all devices are connected, letting the device choose when to swap networks. A thirty quid Archer C50 is the lowest cost option, a bit basic and it won't give speeds above 100 Mbps, on the other hand that's a whole lot better than 7 Mbps.
Best overall solution for poor coverage for most people would be an entry level mesh system costing around £100, I suspect that's more than you'll want to spend.
* This is easily done, but important - if you connect two routers that are both in router mode they'll fight each other for control of the system, causing a range of weird problems. Advice on how to do this can be found on the TP-Link support site, make sure you've read and understood this before spending money.
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