I don't see why flexibility should be a problem. BTW, the final deposit is usually 10% less booking deposit (49k - 5k)
The way house prices are nowadays, most pepole would struggle to get 10% up front. Ask your solicitor to negotiate for the lower deposit (ie 20k or so). This deposit is really just a 'goodwill' figure, showing you are committed to the purchase, as this is what you lose if you pull out after contract signing.
I think its better for the vendor to take a lower deposit and guarentee contract signing rather than lose the sale just because the upfront deposit is too high ?