Does anyone have advice on when to stop cordon tomatoes?

I am growing Sungold and Summer Sweet outdoors in Hampshire, UK. (& Big Boy in a cold greenhouse)

I start them early and we've just started eating the first ripe Sungold & Summer Sweet this week, not many but still pleased considering the recent weather.

The advice seems to be to stop them after 4 - 6 trusses...

I have open flowers and small fruit set upto & including the 6th truss & can see the 8th truss with flowers not yet open.

Bearing in mind it's not yet July, is it really necessary to stop them now to allow them time to ripen.

Is there a date based method, after which there will not be time for the tomatoes to ripen? Last year we did not get a frost until November, which leaves possibly 4 months.

Are there other valid reasons for stopping them now?

I see on other posts stories of 8ft tall plants, and 2000 cherry tomatoes etc.. these surely have not been stopped at the 6th truss.

Also any pointers on stripping the lower leaves

Thanks in advance

Gary