Current members of the British Gas/National Grid pension scheme may join for an annual fee of £ 1 which is deducted from your pension; National Grid add a subsidy to this.
The Member Application Form can be downloaded here
Current members of another British Gas REA can transfer to the London HQ Branch.
This Transfer Form can be downloaded here.
Those who have worked at British Gas London HQ but are not a member of the British Gas/National Grid pension scheme may join as an Associate Member for £ 6 per year.
This Associate Membership Form can be downloaded here