In short it means that the lender you have (presumably) chosen is prepared to lend you £x based on the information so far provided - subject to certain conditions. If you havent done so already you'll be asked to provide things like Passport as id, bank statements etc etc and as long as the info you have provided through the broker is correct and any further checks the lender does come up clean, you'll be made a formal offer which is valid usually for 6 months.
So in short, no it doesnt guarrantee you the funds, but providing you've been honest with the application then it should (and i mean should not will!) be a formality.
As to how long the process takes - how long is a piece of string. Depends on the lender and how busy they are. This said, I'm not sure exactly what bit of the process youre talking about - if its just getting something agreed in principle it should be almost immediate. Its the next part leading to the formal offer that can take longer.
Having just re-read your post it appears that you havent actually made an application yet - so yes this will mean filling out a full application form, so be honest.
I'm sure one of the IFAs here can provide more detailed info as they deal with these on a daily basis.