2 Jun 1972 according to the website you will reach by clicking here. Once there, scroll down to the paragraph which opens, "Construction of two prototypes..."
Its quite confusing when you start reading up on it but it looks like the first British scheduled flight seems to have been on 21st January 1976 when BA flew London-Bahrain using aircraft number 206.
The 002 prototype with Brian Trubshaw first flew from Filton in April 1969 (it says here). I suppose that this was as close to a British prototype as it gets.
The first production aircraft flew in December 1973
There's a vast difference between a 'maiden flight' and a 'first scheduled flight'. It seems that Merlin's source suggests the British maiden flight - which is what you asked about - occurred in 1969, whilst my source suggests that happened in 1972.
I am, therefore, mystified as to why you have chosen 1973! All-in-all, I'd imagine Janes is more reliable than Wikipedia in aircraft matters, so 1969 is probably the best bet. But what the hey!
Trouble is, there are always so many firsts in this sort of thing and when it is a joint venture between various Countries it get even more complicated. I think between us we covered most options.