It's the fact that the house might go down in price that's preventing you from getting a 100% mortgage (or anything close to it).
Let's suppose that a bank lent you £130,000 to buy the house but, shortly afterwards, the housing market collapsed and your house was only worth £100,000. If you then lost your job and the house was repossessed, the bank would only get that £100,000 back. You'd still owe them £30,000 but the bank would have virtually no chance of getting it from you, so they'd be massively out of pocket.
In the past, banks gambled on the housing market always going up (or, at worst, remaining stable). So they were prepared to offer 100% mortgages. Then everything went wrong and the banks made massive losses, forcing some into bankruptcy and many others into seeking government help. That's why we're still struggling to come out of a recession and the UK economy is nearly bankrupt.
As a consequence of the world-wide economic collapse caused by the rash decisions of bankers, all banks are now far more cautious about lending money. That's not solely a decision for bankers. If any UK bank was to now offer loans which exceeded what the value of the property might later fall to, the Government (through the Financial Services Authority) would almost certainly seek to withdraw that bank's licence to operate.
So 100% mortgages (or anything approaching that figure) have now gone forever. In order to guarantee that they'll never make a loss, most banks would prefer mortgage applicants to have a deposit of at least 20% (because house prices might easily fall by that much, if not more, in the event of a 'double dip' in the economy). But they can't exclude too many borrowers (otherwise they won't make any profits), so they might consider 15% or, if the applicant is in a particularly secure job, possibly 10%.
Chris