Pretty bad. Would be interested in the other side of the story.
Quite obviously a great many people with Israeli stamps in the passports enter UK all the time.
Its possible that one could be denied entry to the UK for giving lip to the customs agents. They can be pretty abrasive at Heathrow, and have full discretion to deny entry
to a smartarse if they feel like it.
From what I can gather from other sources, the other side may be that he was coming there to work for the summer and didn't have a work visa.