If you are asked for proof that your foreign marriage is recognised by the British authorities in order to use the Surinder Singh method to get your partner into the UK. i.e. The British Citizen has been asked for proof of their marriage registration in the UK to confirm that their marriage is genuine, when attempting to exercise their freedom of movement within the EU. You hit a bit of a brick wall as the UK no longer offer this services. The only thing you can do is get a standard letter from the embassy stating that fact. If you want it personalised it apparently costs £45.
This link tells you about a saga getting the government to admit this
This the gov being a bit more helpful
and this is an example of one
This is the wording
To whom it may concern
Marriage in the UK is governed by separate and different legislation in England and
Wales, Scotland and Northern Ireland. None of the Register Offices in England and
Wales, Scotland and Northern Ireland issue certificates of marital status.
Only the courts in England and Wales can recognise a marriage under English and
Welsh law which has taken place in a foreign jurisdiction. Whether the court will
recognise such a marriage depends on two independent factors being satisfied
separately: the parties must have capacity to marry and they must comply with the
form of marriage. Capacity to marry is governed by the law of each party’s domicile.
The usual rule in terms of the form of marriage is that if a marriage is valid under
local law, the marriage will be recognised in English and Welsh law. If the use of the
local form of marriage is impossible, the marriage will be recognised if the marriage
is celebrated in accordance with the requirements of the English and Welsh common
law. A matter which goes to both capacity and form is consent. No marriage is valid
if, by the law of their either party’s domicile, one party does not consent to marry the
other. UK marriage certificates are not amended or updated after the couple are
Prepared by Consular section, British Embassy