The capacity of most exchanges themselves is actually increasing. Eventually, they'll all have to carry fibre-optic broadband, which is a completely different level of traffic. Some of them will also serve leased-lines; dedicated cabling that goes directly from the exchange to a customer's property and can deliver speeds at network level (100Mb/ps and above). As I'm guessing that you don't have either of the above, I would suggest that it's probably the copper network in your local area. The copper network is operated by Openreach and the service all service providers. If the issue is with the local network and the lines in the area, then Openreach would have to decide whether or not it was cost effective to replace the affected cabling. As they are currently investing £2.4 billion in fibre-optic technology, they may not be in position to make a degrading copper service a priority.
On the bright side, there may still be something you can do; make BT aware of your problems and also ask your neighbours to do the same. All of this gets back to Openreach eventually. The best outcome is that there is a repairable fault on the network affecting more than one user. If it can be resolved, you'll all be up to speed in a few days. Secondly, your service providers may have taken protective measures on the line. Broadband is an adaptable service and as such slows down where a service is intermittent. Service providers sometimes proactively 'band' the speed profile on the line to prevent it from getting to the point where it might start being intermittent. This is usually in an area where a degredation on the line is already known and the indicative line quality is 'red'.
Failing all of this, if an issue is identified, but Openreach aren't willing to fix it, continue to raise faults and ask your neighbours to do the same. At the very least, it might prompt them to make enabling fibre in the area a priority. There don't seem to be any plans in the area at the moment, but the Llantwit Major and Barry exchanges already are, so you might not be a million miles away. Keep an eye on it here.
Hope this helps.
