I assume you are using Windows?
at a command prompt.
Type tracert mydonate.bt.com
I can access the site with no problems. DNS lookup gives 18.104.22.168 for mydonate.bt.com
That is a BT owned IP address.
Tracing route to mydonate.bt.com [22.214.171.124]
over a maximum of 30 hops:
1 <1 ms <1 ms <1 ms 192.168.1.1
2 * * * Request timed out.
3 * * * Request timed out.
4 17 ms 12 ms 14 ms 126.96.36.199
5 13 ms 12 ms 13 ms core1-hu0-15-0-7.ukcore.bt.net [188.8.131.52]
6 13 ms 12 ms 12 ms core3-hu0-1-0-0.faraday.ukcore.bt.net [184.108.40.206]
7 13 ms 12 ms 12 ms acc1-te0-0-0-9.l-far.21cn-ipp.bt.net [220.127.116.11]
8 12 ms 12 ms 13 ms www1.campus.bt.com [18.104.22.168]
9 * * * Request timed out.
10 * * * Request timed out.
11 * * * Request timed out.
12 * * * Request timed out.
13 * * * Request timed out.
It must get redirected at www1.campus.bt.com which is where the trace ends. So in theory you should end up at the same endpoint.
I seem to have ended up at the same place www1.campus.bt.com. Still no joy via a browser though
I have tried Chrome, Firefox, Edge and IE11.
Then I have run out of ideas. As its a HTTPS issue, it must be a misconfigured network router somewhere.
I am on ADSL, and my routing is quite a bit different to yours.
Perhaps a couple of other forum members, who are on VDSL, can try the website?
Is there anyway you can try it while connected to your BT Wifi/FON connection, as that takes a totally different route?
It could be an issue with the home hub, or an issue at the fibre cabinet port/routing.
Have you another home hub or router you can try?