Some phones have pretty weak Wi-Fi antennas, so that's another thing to throw into the mix that can result in phones and tablets getting pretty poor speeds and very high latency. Oh and where you sit in relation to the access point, how many other people are using it etc.
It's a minefield.
My current phone has 802.11ac and can connect at upto 866Mbps, although that doesn't mean the equipment I'm connecting to can!
Personally, I favour using my own data connection. As said by others, Three and EE are way out in front for mobile data (and the required backhaul) but Vodafone and O2 are catching up and in some places may match or potentially beat EE/Three.
A portable hotspot is a good idea as you can position it by a window and leave it there, but few operate at 5GHz which I'd consider a must these days. Huawei do one but it's much more expensive - but worth it in my opinion. 802.11ac and 300Mbps 4G support, so it's fairly well future proofed. Yes, EE is testing 600Mbps 4G currently but there's still a 150Mbps cap in place for all non business tariffs.