There is now a website where one may sign up to join.
See:-
http://www.familymobile.co.uk/To be a customer of Family Mobile you do have to be a member of IKEA FAMILY. However, when you sign-up for Family Mobile, you will automatically be registered as a member of IKEA FAMILY. It's free to join, and being a member of IKEA FAMILY gives you lots of free benefits, special promotions and other great offers from IKEA.
Their website has a FAQ section:-
Let us help you
> Can I keep my existing mobile number?
> How do I activate my new SIM card?
> How does Voicemail work?
> Calling while abroad, or 'Roaming'
> How do I sign up with Family Mobile?
> How do I know if my handset is locked?
> Is there a notice period?
Features of special note include:-
You can pick a number.
Refer a friend, get £10 credit.
Minimum top up £10.
You can only top up by a valid UK debit card.
Offers call divert. 15p a minute. *
Unique on Pay & go*.
Cheapish calls to 0844/0845 as far as a mobile is concerned: 9p a minute.
First minute charged in full, then second billing operates.
Offers Web Text which allows you to send text messages from your mobile number to any UK mobile from your computer.
Verdict:- Probably worth taking a look !