There is a +44 number for overseas callers listed in the Admiral welcome pack that contains the Policy Certificate etc.
I called this number after becoming a customer in October to discuss provision of proof of bonus (I actually bought the policy online at what I can only describe as an incredible price that was £200 less than any competitor and with very unusual Guaranteed bonus cover despite my 2 claims in the last 3 years). The lady answering the call raised no objection to handling an admin query of this kind. They also looked up my bonus via an online link to AXA's database (or some central sharing database for major insurance cos) and said there was now no need to send in my renewal notice from AXA.
The +44 number is quoted as being from overseas so they clearly expect policyholders who have taken their car abroad to call about claims. I also get the impression of Admiral staff being generally multi skilled and multi tasking and when, on completion of my call, I congratulated them on publishing a +44 number they didn't claim I should have been calling 0870 instead.
So whilst Admiral may quote an 0870 number as the main contact number in documentation they seem to me to make it as easy as possible to find an alternative. They also list 029 XXXX XXXX for Admiral Group PLC with Directory Enquiries too. Plus of course the Freephone number of 0800 600800 for sales. And they have some of the best prices for car insurance too. Come on give these nice Welsh people a break guys.
Edit - 029 number removed as no longer valid.