A couple of users on this forum use AA (AA.net.uk) as their ISP, I am one of them. They can tie multiple lines together for you to make a truly n times faster pipe, in the sense that even a single transfer made by one user is n times faster, in both directions. The lines don't need to be of identical speeds, they combine up to give their true sum, not just n times the slowest. The lines can be over different carriers.
I am notorious for recommending them a lot, hence the joke earlier, but I have no commercial connection with them other than just being a very happy user.
Unfortunately they are not cheap, but you get what you pay for in that they are obsessed with giving the maximum speed with no compromise and this costs.