Hello, last month I was flying home to the UK from Dubai on Air France (DXB-CDG-EDI) in Premium Economy, it was the return leg of the ticket. Premium Economy was overbooked and upon check-in in Dubai I was downgraded to Economy. The check-in agent told me I was entitled to a GBP/EUR 150.00 voucher (he wasn’t really clear about the currency, I later found out it was instead AED 600.00 which is roughly the same amount), and to 50% compensation of the price difference between Premium Economy and Economy. They asked for my email address but I never received any voucher.
They never said I could choose between voucher and cash, or a combination of both.

Arrived in Paris I went to the customer service desk where they could see the voucher but couldn’t print it, they instead took a picture of their screen showing the voucher of AED 600.00 and told me to submit a claim to retrieve it since I was also entitled to a 50% compensation of the ticket price.

Arrived home I started researching about my rights and according to EC261 I am entitled to 75% of the ticket price.

I submitted a claim through the AF website claiming for the GBP/EUR 150.00 voucher, the 75% compensation, and the refund of the seat selection.

The day after I receive an email saying the refund request has been submitted, but in the pdf document the ticket number showing (starting 074) was the one of the AED 600.00 voucher and that I would be refunded by credit card (which hasn’t happened yet anyway).
I then sent an email saying that I wasn’t asking for a refund of the voucher, but for the actual voucher, the EC261 compensation, and for the refund of the seat selection.

A few days after they replied that they could process the refund request, “as the concerning document was issued as compensation in the form of a voucher”. To exchange the voucher for cash I would have to submit the relevant form online.
I then replied with a quote long email explaining everything again, citing EC261, and threatening to bring the matter to court, but three weeks have passed and no reply has arrived, even though I’m chasing them every week.

They have been very quick in replying at the beginning (for rejections of course) but everything I get now is silence.
I would like to file a formal complaint to the National Enforcement Body but I don’t understand which is the relevant one for me. From what I’ve read the flight should be covered by EC261, being a flight on an EU carrier from non-EU to EU.
But to which body should I file a complaint being a UK resident?

Thanks in advance for your help, I tried reading past threads and messages but couldn’t find a relevant answer, also considering Brexit came into force only two years ago.