Oh well – the same problem as Vincent's has hit me also today. Browsing through the answers, I don't see what cured it for him, finally (I did not tick 'rest of the world' !).
Trying to subscribe via the button on this webpage results in the same error message, but the price is about 2£ higher…
>>> Sorry, we are currently unable to complete your order because either we don't ship to your country or you have products with mixed payment types in your basket.
Please call Customer Services on 0844 543 0064 (UK), +44 1604 251043 (International) to complete your order.
<<<
Nasty, really, as I have MEW since issue 0 (or maybe 3),. And mixed payment types? what would that be?
First thought was : oh, another Opera problem. So next try with Firefox, but again the same.
As I think I would hardly understand someone over the phone, I think my next option would be filling out the form and make a visit on the post office.
How progress is wonderful – I can't remember when I had to do that last time.
Regards, HansR.