Is it legal for a 21 year old male to date an 18 year old female in England?

Yes. The age of consent in England is (as far as I know) 18 years old. If all else fails, use the rule of 'Half-your-age-plus-seven', which illustrates what the minimum age of your partner should be to have a 'socially acceptable' relationship.

A.
No actual law exists about dating,

The age of consent in England and Wales is 16

However it is illegal for a person to engage in sexual activity with an individual under the age of 18 if they are in a position of trust in relation to that individual like teacher, warder, care giver, guardian, etc.

So in short It is legal in your case.