You should be able to dial a router Internationally via ISDN. The important question is can your telco communicate with the UK telco. When you connect a call you are essentially connecting to your local switch which provides the D-channel signalling to your ISDN device. Once it reaches your telco's switch it is up to them to route your signals to UK. The UK telco should then send d-channel signal to the ISDN device in the UK using UK's protocols and switch types. They are not the same by the way. Here in the US AT&T, NTI and NI-1 are the most common. In Europe they use Net-3.
Also you need to make sure that your ISDN line is provisioned to call an International number. It may not be. I've seen instances where someone wasn't even provisioned for domestic long distance.
Long story short, it is up to your telco to make that happen. At least that's how I understand it.