Heh ye they used to be crazy prices. But yes to a business £60 is nothing, in fact when you make me look upon it like that £60 is nothing. £30 even less so - at home you’re comparing it to Sky etc so it seems a lot.
With the right router you could get them 2x 4G services in, make the second one Three as a warm failover.
Good luck with it all, hope it works out, and I hope they appreciate your efforts!!