From my experience mohdd, the lost likely use is in attempting to obtain a mortgage fraudulently and you'd be surprised at how severely such cases are punished by the courts in the UK. Attempting what is a fraud against a financial institution, which could lose them money, means that they will press for a sentence that makes the perpetrator regret it. Of course, you'll also lose the mortgage, and the property through forced sale.
Also bear in mind
http://www.fca.org.uk/news/firms/tribunal-upholds-80000-fine-for-broker-who-lied-about-incomeOn top of everything else, he also lost his livelihood.
https://www.lovemoney.com/news/12923/why-mortgage-lenders-can-see-your-tax-returnTHE MORTGAGE VERIFICATION SCHEME
The Mortgage Verification Scheme was announced in the March 2010 budget and was put together by HMRC, the Council of Mortgage Lenders and the Building Societies Association. All three organisations see it as an important tool to beat fraud.
Now the scheme is up and running, anyone tempted to inflate their salary or submit fake documents (such as pay slips or bank statements) to their mortgage lender should be aware that if they’re caught out they could find themselves being investigated and possibly prosecuted for fraud.
The only people that need to be worried about their mortgage application being verified this way are those telling porkies about how much they earn.
In the best case scenario, you'll just be blacklisted from ever obtaining a mortgage in the UK.
Anyone thinking of defrauding a lender better have something better than just a fake document ordered off the internet.
The only valid 'replacement' payslip is one issued by your employer, or a copy thereof, otherwise it is a facsimile and should be used as one and stated to be such. Nothing illegal has happened until the document is 'passed off' as something it isn't, so the website is doing nothing illegal. It is the person doing the 'passing off' who will be prosecuted.
It's unlikely to work but, if it does, then you're in serious trouble.