First thing to do is get your mortgage in principal sorted, before even looking for a property!
Even then it doesn't mean much these days.
I had a mortgage in "principle" sorted in May, placed an offer which was accepted, put in 25% deposit on a 300k house, and then the b******s at the bank decided they needed more deposit off me, which I couldn't have provided at the time.

Lost my dream house, just because the bank decided that it was too risky?!
I had the perfect credit score, little outgoings and savings to show it. And despite my rather large deposit, they wanted more!

It's not always the Banks either - Just before I got married, I had a Mortgage arranged, put down an offer for a town house (I had plans to convert the ground floor garage into a studio flat - would've covered 90% of my repayment!). My offer was accepted. The property was new in the market, I was apparently one of the first amongst a few people to view the property.
The estate agents were keen to refer me to a mortgage provider, I listened to what they had to offer, it was no way near what I had already arranged previously, so I told them that I will sort out my mortgage. I don't think that went down too well with them. When my surveyors tried to gain access to the property the agents turned around and told them that my offer was placed on hold.
When I contacted the agents, they told me that a guy placed an offer a month prior to me and apparently he wanted to go ahead - Lie! They apologised and told me although my offer was accepted, the landlord felt obliged to sell to the other person since he made an offer before me. I put myself in the landlord position, if I was selling a house what would be most important to me? Money and a quick sale! ... And so I offered an additional 10K for the property but the agents were reluctant to do anything, they were not even willing to convey my proposal to the landlord.
I had little time before I was getting married, and so out of haste I placed an offer on another property not as good but I needed a place before I got married. Again, the agents wanted me to arrange a mortgage through them but I told them that I have one already arranged. This time around when my surveyors requested access to the property they were told the place was occupied by squatters and were unable to gain access.
- This was yet another lie by the agents, because I sent a friend to arrange an appointment to view the property, he too made an offer, slightly higher than what I did, they invited him back to the office to arrange a mortgage he too told them that he already has access to funds for the purchase. Few days later he was told the property was taken off the market due to squatters gaining access!