There's a lot of no frills gyms now, so you can do that for £10-20 pm if you want to and there's one nearby.
The amount you were paying for insurance looks very expensive, you can get it for around half that.
Food is quite a bit more expensive than it was 10 years ago, but now or then £600 is a lot to spend for a family of 4, so you must have been shopping in expensive supermarkets and buying expensive things. Aldi and Lidl are a lot more widespread than they used to be and their ranges have expanded too.
A lot of your amounts are a bit 'how long is a piece of string' because costs vary depending on where you live and whether you shop around for insurance etc, cook from scratch from Aldi will obviously be much much cheaper than Waitrose ready meals.
Apart from the NMW increasing quite a bit percentage wise, wages haven't gone up hugely and sometimes have fallen and a lot of jobs aren't as secure as they were (zero hours contracts etc), it's probably more that you have to see what is available and what you are qualified for.
When you lived here before, were you tied to a particular area? Unless you 'have' to live in London or SE England, I would really suggest thinking about elsewhere, such as in and around other UK cities, as housing is so much cheaper so you will have a better quality of life on lower incomes.