Pwsimerimew - I am an independent soc worker after having retired some years ago after 25 years in fostering and adoption for a local authority. I now work part time assessing foster carers and adoptors. There is as you probably know a great shortage of foster carers so glad to hear you are considering this.
Your sw is quite right to advise you to pull out at any time you don't feel this is right for you. You both need to be in it together for it to work, but it's also quite normal for people to be uncertain as to whether it's right for them. I am never worried about this - in fact the opposite worries me more, i.e. when prospective carers are adamant that it will all be fine etc because I don't think this is realistic.
Remember the process is a 2 way thing - yes you will need to be assessed but you also need to work out if you think it's something that's right for your family. As far as the a/ment goes, it is extremely comprehensive for obvious reasons. If you carry on in the process you will be invited to a preparation course where you will learn about all aspects of fostering.
The whole process should be open and you can have info as to what the assessment involves. Different l.a.s use slightly different versions but they all follow the Form F (produced by British Agencies for Fostering & Adoption BAAF) and you can find info about this and other things on their website. You might also like to look at Fostering Network website (the national organisation for fostering)
Imisssleeping has mentioned respite fostering and I think this is a good way to start off, especially if you remain uncertain about whether this is right for you. Ask you sw to explain about this as it allows you to "dip your toe" in the water without making the committment needed for other kinds of fostering and you can always move to short term fostering at a later date if you so wish.
Good luck and happy to help further if necessary