You asked how people book holidays so far in advance, that is how.
The flights are technically booked, you can choose your seats as soon as your deposit is paid. Any other travel company who doesnt own their own airline offering a hotel and flights package? Yeah, those are just guesstimates and not booked till 11 months, and run the risk of high price hikes or ridiculus changes.
Oh no, another country gets an offer you can't use. There are also offers that can't be used by UK guests (or there have been in the past).
The UK (and EU) market also spends more time (and money) there than most US guests. Most holidays are 2 weeks or more, obviously some less, which is why the UK is offered 14 night tickets both at Disney and Universal, and they are often comparable to the cost of a US based 3 or 4 day ticket. For some, it is also a once im a life time and very expensive holiday and they have to budget for it, so getting a price locked in as far out as possible helps them with that. Even then people might not book (as in money paid) but want the information of plan more accurately.
I'm not saying people from the UK don't book short notice, you just seem very disbelieving (and I agree with the other commenter, judgemental) that some people do. As stated before, there are many reasons as to why people would.
Another example: My family tends to book flights 11 months out because they use their rewards point for flights and the longer they wait, the less choice they potentially have. They almost always have the villa they want to stay in booked already, the times they havent booked in advance they've had to do split stays to patchwork where they are staying because the villa they want isnt available the whole time. Once the owner even booked them in for the wrong year, they realised about 5 months before when they went to see if their check out day was still unbooked to extend a night, they then struggled to find somewhere else that didnt mean moving around more times then its worth, or extortionate hotel prices because its over a US holiday period.