Lily - can't help with regard to London clinics, but can't believe you were turned down even with DE for AARGC.
As I'm based in the north, I had 2 failed cycles at LWC Darlington - with double donation, as I'm single.
I changed clinic and went to Sheffield CARE and couldn't fault them - I did double donation again and this time had success on fresh cycle, DS is now 2 years old, got 3 frosties from that cycle. I then had FET this year and had DD in October. I also have 2 frosties in storage.
I was 45 when had failed cycles, 46 when started treatment at CARE, and gave birth to DS at 47, had tx this time at 48 and gave birth to DD aged 49. So I think most UK clinics will treat women up to 50, but usually with DE. With DE, as long as you don't have other problems, the success rate is higher and can be about 50%, whatever your age.