Once we decided that we wanted to move house and the area we wanted to move to, this is the way I did things:
Put our house on the market, as we needed the funds from the sale of our house to purchase our new house and also alot of estate agents will not deal with you when you are looking at houses if you have not got your house sale agreed, never mind not even on the market.
Got in touch with mortgage mortgage broker in order to determine how much of a mortgage we could get, so then we knew exactly what budget we had when looking at houses. Got mortgage approval. You may as well get your mortgage approved as soon as possible but if you dont buy house after 6 months of loan approval then you will just have to submit recent bank statements and wage slips to renew the mortgage again.
I went with a mortgage broker as you just give in all the relevant paperwork to them and they sort everthing out. You can ask them for a list of the banks that they deal with.
Hope that is of some help.