Like you we have an appalling mobile/mobile internet signal but have USB aerials for the laptops (no use for Ipad etc), so the only enhancement we are seriously considering (like that word means you are not spending money but enhancing your holiday home) is satellite broadband which is being covered in a thread by the well known guinea pig Angela
The aerials are great 100% signal strength although unfortunately it is the distance from the exchange to the park and number of users that dictates our speed. We are probably at the end of the line from the local exchange so no aerial is ever going to speed the internet up only help to connect to the WiFi router.