Have a look at what the signal levels are like with Three in your area, they offer a contract that is unlimited* everything (minutes / texts / data) for around the £20 mark, their phone sims are also no longer prevented from working in dongles / mifi units.
* we all know that unlimited deals on anything really have limits, previously Three used to start looking at your usage when you went over 1000 Gb in a month.
With regard to landlines we have previously had a landline installed to a wooden shed on the beach, BT will install almost anywhere but you will pay through the nose for everything (initial survey / installation / line rental), ask about non served premises, in our case it worked out at around £70 per month for line & broadband but we were probably an outlier.