Any advice re screen time fir year 11s and how to keep the noses of lazy but bright kids to the grindstone without making them stressed about GCSEs? [Edited by MNHQ at OP's request]

Have it in a common area.
Only allow it for tasks he needs it for.
At least until he proves he can be trusted.

Instead of having to threaten to take it off him, promise some time when he has done his work.
Encourage him to make a timetable of revision and learning for each subject. If you can help him designate a certain amount per subject depending on how much work is required.

Might not work for you OP, but with DD1 she had an upstairs laptop for h/w which was shut down tight with parental controls. Then downstairs she had a more open access. So we knew if she was upstairs 'doing h/w' she at least wasn't gaming or on SM.

Or, a teacher on here once posted the key combination to recover closed tabs on a web browser. Try CTRL+SHIFT+T next time he quickly closes something.

Surely “most” homework can be done without a laptop? Not all homes have them.

Yes of course anyone can visit Oxford, either at an Open Day or just for a wander around. Lots of uni's run Open Days this time of year, why not go along to one to see if it does the trick. Certainly worked for my DS who is now really focused on doing well to get into his chosen course.

8s are also A*s OP!

So he doesn’t use the laptop or he uses it in a shared space.
A nine is not an A @Witchend. It’s an incredibly high grade and the content of the new GCSEs is very different from the old courses.
My eldest has just finished GCSE courses. I expected him to work hard and meet deadlines. I didn’t expect him to flog himself to death in an attempt to meet the very highest grades.

Depends what he wants to do if he "needs them".

But I think it's silly to say why does he "need" high grades. He might not "need" them but he will have better options if he gets them. Why does anyone need them? If we all race to the bottom then what good does that do anyone?
He may have ambitions now that don't need them, but later regret it. Far harder to go back. Ever heard someone saying "I regret working for my A in GCSE maths?" No, me neither.
Ever heard someone saying "I need to work and get my GCSE maths to do something I now really want to do, wish I hadn't messed around at school"? Yes, on here and in RL many times.

I remember a thread on another forum about learning a musical instrument and whether parents should encourage practice. It was full of people saying that they'd drifted along and stopped and wished they hadn't. The majority wished that their parents had nagged or insisted they practiced.
None of these people "needed" to play a musical instrument, but in a thread of into the hundreds of replies there were perhaps one or two saying that they hadn't appreciated their parents pushing them to practice.

OP, I agree. In my dc's case it isn't overnight that's any issue. It's the "rush through my homework to get back to my game" or "do my homework with minimal effort because I'm mostly concentrating on talking on Discord."
One of the issues is that such a large proportion of homework is on or needs the computer. It means they've got the temptation right in front of them. It's "just a quick game, only take 20 minutes" and three hours later, they realise they haven't gone back to the homework and it's bedtime. Done the equivalent myself on MN.
I've given dd a week to show me that she can work in her bedroom without me nagging. So far this evening I have a strong suspicion that she hasn't even done one, she was warned at 3:30, it's now over 2 hours later.

By next week I suspect she will be working where I can see her, as otherwise she never does anything she doesn't have to, and what she does do is with the minimum of effort.

Op, no idea why you’re getting so many snide answers!

Watching with interest as my disinterested but bright son goes into y10

If he needs motivating consider taking a tour of a university - stuning frescos and marbled floors of Oxford really motivated me at 15 to bust a gut with my GCSE's.

Also contact school and make sure you know of all the after school revision sessions and make sure he attends

The trouble is what works for one child will be ineffective for another. You can only provide the framework.

I failed miserably with first DC, did moderately well with the second and the third did very well.

However the first DC caught up massively when he spotted how others had done in comparison, went on to narrowly miss a First, and won prizes.
Lazy-but-able boys are crucifyingly annoying. Best of luck.

Who says he’s capable? Do staff agree? so He doesn’t “need” them?

Rows? If he argues he can lose all screens for a few days. Time to earn the privileges. Why does he”need” such high grades?

but will that put too much pressure on him?

Well, if you think he is lazy, a bit of pressure would be a good thing, wouldn't it?

Surely if he's bright he should understand the importance of preparing for exams. If he can't be bothered, it might do him good to not get the grades he could have achieved.

Have you had discussion on what he wants to achieve after GCSEs, e.g. A levels, uni and beyond? If he has high aspirations, have you discussed how doing well on GCSEs will set him up for A levels, and/or that some unis will look at GCSE results when making offers? Or even that X school he wants for 6th form needs 7s&8s as entrance requirements.

Personally, I think that though many y11s will be self regulating re tech, many others won't, and y11 is not the year to take the risk if you are at all in doubt.