I have that problem at times too. At a practical and affordable level there is no solution. There are solutions that can work where there's poor coverage inside a building or caravan but some signal immediately outside, even so for a system that works and will be compliant with the strict standards that are applied to radio systems you're talking many hundreds of pounds, and you'd need to speak to a reputable specialist to get advice. It's worth speaking to them if you're prepared to consider spending the necessary money on a high gain antenna and appropriate booster, plus putting the effort into the necessary quality of installation. Anything off eBay or Amazon either won't work, or won't be legal (and the penalties for illegal operation are significant). Ofcom are consulting on making small booster systems (picocells) more readily available to consumers, but that won't be effective until next year and without the (costly) specialist knowledge probably still wouldn't work for this situation.
If there's any other network that does have a routinely usable signal at the caravan, then there's a range of cheapish 4G mobile routers you can buy, and you could search for a data only PAYG SIM on that network for the router (noting the need to keep credit topped up if not in use for extended periods), and that's as good as it gets for the scenario you have, I'm afraid.
