This is a bit of a late reply, but I thought it might be helpful to clarify anyway. You can both go on the birth certificate if:
1) You are civil partners (or married overseas) at the time you conceive - this applies as long as you conceive artificially, and so covers home insemination as well as clinic treatment (i.e. anything which is not conceiving through sex) anywhere in the world.
2) You are not civil partners but you conceive at a licensed clinic in the UK and, before you do, both sign the relevant 'parenthood election' forms (currently HFEA Forms WP and PP). This is restricted to treatment at a clinic, and to treatment in the UK. If you conceive by home insemination or abroad then you won't both be parents automatically.
These rules came into effect for conceptions on or after 6 April 2009 - before that only the birth mum was a legal parent.
There's more info about all this at
http://www.nataliegambleassociates.co.uk/knowledge-centre/donor-conception-law-for-lesbian-mums which may be helpful too.
Hope that clears up any confusion though.