The UK phone connector is bigger than the ADSL (RJ11) jack. Lots of info at Wikipedia:
With regards to connecting, say, a UK fax machine to the Dutch phone system - there are differences in the electrical spec and so there are no guarantees it will work - for example, the fax may not detect incoming calls or be able to 'pick up' the line to send. One hope may be if the fax machine has a 'country' setting. Perhaps someone here has practical experience of doing this - I have a Dutch ex-colleague so I'll send her an email and update here later.
Update: My Dutch ex-colleague says "won't work properly" but didn't elaborate.