Get yourself an adviser...there are loads ofcompanies out there who will do self-cert mortgages but you usually need quite a low loan to value like max 80%....so you need at least 20% of the value of the property you which to buy as a deposit......
so if the house is say £100k you need £20k deposit ....mabe even less than that these days, to get a 'normal' rate of interest...anything above, you're very likely to be paying over the odds in interest.
Same goes with remortgaging...you need to retain at least 80% (or whatever the figure is) equity in the property after the amount you have remortgaged for.....HTH