If you're not using a lot of data a mobile phone is a great way to do it. Low end Android phones are cheap and surprisingly powerful. Prepaid phone plans are inexpensive.

I bought a huawei 4g mobile wifi device off ebay for about 25 quid earlier this year, and have a 3 sim card (the huawei device is branded 3 anyway, but they don't lock them) with 20Gb/mo data for 15/mo but you can get cheaper contracts since you don't need that much data. I run it from 12v, pretty sure you could mount it on an electric weelchair..

ee pay as you go offers 100mb for 30 days for £3.

Gonna be hard to beat that, cost-wise, with anything reliable and covering a large area.

You can pick up three sim cards prepaid with 12gb data for about £23 on occasional sales which last 12 months if you don't use the data.