How do I change the default Kindle store associated with my account?
The default Amazon Store will default to the region you are in. So for example you may have a UK account, uk bank, uk bank card and uk amazon account, but if you live in Spain then your default account will always be www.amazon.es or whatever the correct spanish account thing is.
Rollback Post to RevisionRollBack
The 3 standard solutions seem to be RESET / RESTORE FACTORY DEFAULTS / CHECK YOUR REGION.
if you physically live in the UK, Ireland, Gibraltar or the Channel Islands, you have the choice of amazon.co.uk or amazon.com and that's all
if you do not physically live in the UK, you will have the choice of amazon.com and whatever Kindle Store, if one exists, that covers the country in which you are physically located but you cannot switch to amazon.co.uk