I think it's more people than you imagine. Certainly, just thinking of close friends I know - as a disclaimer, I am middle class - I have several friends whose parents contributed a significant amount to the purchase of their first homes. One friend's parents bought her a house in uni so that she could largely live rent free (and take in friends as lodgers). That was 20 years before I could afford to buy a house! So you could imagine the advantages that gave her in terms of equity.
My family gave me money when I bought a house. It was my inheritance for all intents and purposes. Their thought was, why sit it in an investment account somewhere growing money for them for another decade or more when they could see it put to good use while they were still alive? They are financially comfortable with very good private pensions and to be completely honest, have more money than they no what to do with. They have a habit of giving money to friends and family, so I think this was just normal. Now has it caused me no end of drama in the end? Yes. It has, because I have tricky family dynamics. But I'm grateful for the financial security it's meant I could provide for my children.
I've honestly never talked to anyone about it though. Dh and I both earn well and we saved a huge deposit on our own before we bought our first house. So I don't think anyone would think we had some family money in there as well, though maybe they do. For Christmases, birthdays, etc. large monetary gifts would have also been fairly normal. I don't mean, like £5k for a birthday, but yes, I can see how it might add up to £1-2K over a year or so maybe for multiple occasions. Again, it would never occur to me to mention it, nor would it occur to me to ask what other people's parents give them. Certainly, people spend silly amounts on gifts sometimes - a purse, a new iphone, a crazy expensive meal out, but we often think of giving money of the same amount as being somehow privileged.