In answer to your question OP, I would maybe say "better than average".
Yes, 'wealthy' is a matter of perspective, but it is a matter of long term income vs expenditure really, not assets. Owning a home of £700k doesn't help improve your ability to live month to month. It would only help if they sold it and bought a house for half the amount and banked £350,000. Then, I may consider them wealthy.
I know 2 sets of friends in a similar situation, but they often struggle financially as their income is up and down and doesn't always cover their monthly costs. They also have multiple properties, but they are mortgaged. They do have rental income, but it maybe brings in a few hundred per month profit on each property.
Maintenance issues and costs of being landlords sometimes catches them out and they have a 'bad month'. We can all struggle but have vastly different levels of assets.
I have another friend who had £80,000 saved. 2 properties owned at about 50% LTV. Many would consider them wealthy, however, they had a bad year with their business last year and most of the £80,000 was used to simply cover living expense as their income dropped dramatically by more than three quarters.
I can see how your in-laws may struggle some months, but they are simply living outside their means. 1 income and 2 x 2022 cars, plus 2 kids in private school probably takes up £6,000+ per month. So, even a £10,000 per month income could 'struggle' with all of the other bills life throws at us.