Well the thing is nothing is definite until contracts have been exchanged and a completion date has been set. The good thing is that if the seller pulls out of the deal after you've paid all fees/deposit they will have to pay you back as well as a little more for compensation.
We actually put an offer in on a place last year but unfortunately we couldn't get a mortgage due to an unforeseen problem peering its head. When we had that sorted out we looked again and made sure that when we found something, we really really liked it, enough to put a deposit down and go for it.
There will always been something else you like the look of but if you like something enough that you offer a huge amount of money to have, then it's the right decision and don't torture yourself about it.
I would get a mortgage approval sorted out right away and if it falls through, then don't worry there will be something else out there.
