Chloe , I too am sorry to learn of your circumstances .
If you cannot find someone to step in and take on the contract , another option might be to look for an unlimited data SIM card for your mobile ?
It has now been five years for me , but since my iPhone went out of contract , I happened to be walking past a Vodafone outlet and saw that they were offering a SIM with unlimited calls , texts and data for £21 per month ( better deals may now be available ) , when I went in to enquire , I found that because of my employment ( emergency services ) there were further discounts and I ended up paying £17/month .
If you cannot get broadband , you may be able to get a suitable deal which would allow you to either create a hotspot from your phone , or tether it via Bluetooth or USB to a computer , thus getting a connection until you turn 18 .
I've been using my mobile for data for the last two and a half years , since separating from my partner , and am only now getting broadband , having just bought a house . Throughout that period I have typically got circa 250 Kb/s over 4G , not blazingly fast , but reliable and useable .
Just an option that may help in the short term
It is always worth shopping around .