[Sundae]

I'll let a proper historian answer this properly.
Of course.

But to my mind; no.
Sorry.
Apart from the fact that England and the UK are very very different entities, there is no recognisable beginning to these countries. They go back before our recorded history.

Limey lives on an island with remains of many old settlements. Stone Age I think, but she can correct me.

Personally I've seen Stone Age, Iron Age, Bronze Age and Roman remains here. In fact Roman walls are present in many of our big cities. Bath has a spa which was originally Roman and restored sympathetically. You can walk city walls quite easily in many places, or Hadrian's Wall of course, built to keep the warlike tribes of Brittania out of Roman Britain.

Dana might talk about the Magna Carta.
Others about 1066.
Or even the death of Elizabeth I which joined England with Scotland (a link they are trying to sever. Go on then! See if you can manage free prescriptions and student fees without us! Bye-bye, bye-bye...)
Same with Wales (same for me anyway - they get free prescriptions!!!). They joined us after Llywelyn ap Gruffydd died. Owain Glyndŵr made a bit of an effort in the 1400s, but it came to nothing.

But every schoolkid knows Edward I smashed the Welsh. Shame. Apart from them being quite uppity I am very fond of them. I have no reason to claim Celtic blood, but I wish I could.

So, personally, I can't think of a real "start date".
I look forward to more learned people giving their opinions.
I adore history, but am no scholar.