There is an expansion program going on now with BT. Its called "Fibre to the Cabinet" You may have noticed BT engineers working in your area, putting in new green cabinets in your area. ( Don't be confused by vans that say BT Openreach...its just another division within BT ) These new cabs are distinguished from the old ones by being a little bigger and having a yellow fibre optic sticker on them. This program is to upgrade your line from the Exchange to that new cabinet, with fibre optics. Without it, BT Infinity can't be provided to your house. No other company can provide it either, without the fibre optic component.
So, as they have been completing each Exchange area, they are able to offer much faster downloads speeds. Whether its worth it to you, only you can decide. For example, at present I get about 11-12 mb. With Infinity I could get as fast as 76mb. But I don't download films and such, so at present the extra speed is not of much use.
The extra charge that BT make for these higher speeds can be as little as another £5.00 a month, and the installation can cost only a one -off charge of £30. But it could be that there is a special offer in your area for much less than this at the moment.
There is no catch as such. You may find the extra speed very useful, even if you don't download much if any data. I would visit a friend who has Infinity and use their PC for an hour or so, and then you can see if a move to this better service is worth it. Another £5.00 a month doesn't seem very much in this day and age for such a fast service. This progression on to higher speed by fibre optic is probably more significant to us end users than the change from dial up to our present condition.