If a person has a ten-year UK visa with a 'Valid until' date of 1 July, 2018, does anyone have personal experience of whether it means that so long as they enter the UK before 1 July, they can stay beyond that date?  Or does it mean that they have to both enter and leave the UK before that date?

Once the visa expires ie 1st of July 2018, you are in the country illegally.you must leave on or before the date of expiry.This applies to all visas that have a "valid to" date on them.

https://www.gov.uk/standard-visitor-visa

 

https://www.migreat.com/en/how-long-are-long-term-visitor-visas-valid-and-can-i-apply-for-one/

 

"You are only allowed to stay in the UK for up to 180 days during each stay, unless the visa expires before the 180days – as indicated by the ‘valid until’ date shown on the visit visa."

 

 

This is different from other county's visas. where a 'date of expiry' or 'enter before' date may only refer to the visa expiration date and not the limit of length of stay once the document is used when entering the country.