Summer retainer Vs. Subletting for last 3 months of existing AST

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Four Very good tenants now into their 3rd quarter of 1 year agreement (they can break out at 9 months) are interested in staying another year however they won't all be around for 3 months and are asking about possibility of retainer or subletting to their friends.

One of those tenants will stay for about half that summer period, whilst the other three go home.

The options I think are:
A. Reduce the rent just for those 3 months, take back the keys from the other tenants (the keys cannot be copied without me being involved), and charge either a reduced rated between the 4 of them, or 1 of them paying a bigger proportion of it.

In which case, is there anything I need to do. Is it sufficient for me to write and sign a document that just for those 3 months I agree to accept a reduced amount til the expiry of the 1st year contract? Or is there anything else I need to do legally?

B. Charge the same amount as per usual, but let tenants manage their friends i.e. they sublet to them for that 3 month period.

Is that OK legally or would that be jeopardising anything legally or leasewise by doing that? (They're all students and so would prospecitve sublettees so from my point of view, i would inform council of new council tax exempt residents, would there be any other responsibilities?

Thanks for any advice given