If the choice were 75K in full-time employment vs. 100K in contracting, I would take the 75K every time. You get paid holiday, sick leave, pension provision, laptop, stationery, paid business travel, training, etc, etc, etc. Your overall 'salary' is a lot higher than your basic, and there's a decent chance of career progression as well. You also have the certainty of a job in the future and a certain amount of protection (in terms of notice period / redundancy payments / etc) if things go wrong.
The world of contracting is very lucrative, and can be a good move -- but you've got to weigh the pros and cons. First, you have to know that you'll always be a contract away from unemployment. You need to factor in the fact that you won't get paid between contracts, that you'll need to bear your own expenses (certainly those relating to business development etc). And you need to accept that your career will generally stagnate -- you'll be taking on the same sorts of roles again and again instead of building towards something bigger.
For me, there would need to be a substantial premium -- more than 25K -- to make contracting an attractive alternative. At £800 / day, for example, you're talking over £150k per year based on a 200-day year, which suddenly looks attractive. Factor in the ability to shield a lot of that from income tax, and the argument becomes more compelling.
Summary: contracting can be good, but not if the day rates are only 33% over your current salary.