I recommend joining the support group l belong to. Its called The Pernicious Anaemia and B12 Deficiency Support Group founded by Pat Kornic. There's a wealth of information on there. I have learned so much from this amazing group. Mainly that you need a constant level of at least 1000 to feel well and you also need for your folate level to be optimum to make the B12 work properly. B12d is severely undertreated in this country.
To think that London, or anywhere else for that matter, does not belong to any one demographic