I think the title was a generalisation, the example though started at £25k, and as others have said £700 for rent is high if you're earning <=£25k.
However I don't see the point in taking apart the maths and coming out with, "you'd have £450 left, not £400!!". What seems glaringly obvious to me is, the OP is very conscious of earning a decent wage to get some of the nicer things in life. I base this on the round number of 30k, the general question subject and the use of luxury lifestyle.
The reality is, if you are concerned with building a career with money in mind and not doing something because you love it et cetera, then there's a couple things. Firstly, £30k is by no stretch of the imagination a lot of money for someone degree educated, it's just not. Many graduates start on this kind of money, and I took 6 months in my first role before I was earning £30k - I'm by no means special (£28k to be precise).
Secondly, you're not going to spend your life on your first wage of £25k, especially if you're driven by money.
Depending on what field you work in, it may be the norm to move around on a fairly regular basis and jump the pay scale each time in the region of £5k. Now this is a large generalisation so to reiterate, it depends on where you are and what you do and what industry you do that in. For example let's say you graduate with Computer Science, you work in the North of England and for a charity. You're not going to earn the money a CS graduate would earn working in Aberdeen in the Oil & Gas industry, or London/Edinburgh in finance.
In a nutshell, your first few years of work are usually far from luxurious. You're working normally to gain experience, you're not quite in your dream job nor is your lifestyle. You may be living with your parents, in a dive or house sharing with randoms. You may drive a crap car, you may fly economy once a year to Costa del England.
The point is, in 5 years if you've knuckled down, you won't be earning bad money - a fair bit more than 30k - and you'll be looking at more senior roles with some potentially nice bonuses and employment packages. Compare that to 5 years on the dole...