It looks like you would prefer to do commercial modelling so you need to approach appropriate agencies one you are happy you have a good selection of images to show them. They are looking for natural looking lifestyle shots, in non-logo clothing
I know the above poster was harsh and he really should have followed it up with the reasons why you might struggle.
Here is a couple of agencies I have heard of (I can't vouch for them all but I certainly wont say I have heard any negatives about them).
http://www.sandrareynolds.co.uk/recruitment.aspx - You may fit into their "real people" section but not fashion or "classic" models.
http://www.gingersnap.co.uk/models/list/106/all/1?cs=w3 After a quick look I can't see any models under 5'5 in either their main board or commercial board. From that quick look though there are a couple of ladies on their "classic" women board which are 5'4.
Heres a tip I found online which answers the question "how do I become an avon model?" http://www.ehow.com/how_4422752_become-avon-model.html
Chances are they use one of the main agencies and the chances are those agencies have strict criteria when it comes to age, height and looks. Lets face it if a model is signed to a good agency and sent to all these castings then chances are it will be a tall model that is selected (even if height isn't a requirement for a headshot).
I know its hard to "give up" on a dream but I would say that it might save time/heartache not to chase the bigger agencies or companies because the possibility of being shot for a huge campaign and signed to a model agency isn't really that high for you (just like it isn't for most of us) and I know its tough to come to terms with that but for me anyway I knew I wasn't fashion model material even when I was 22 and stood at 6'1 tall so I explored avenues that were open to me. Maybe sign up for the website starnow (which is similar to this site but mainly focuses on the acting side of things). Maybe with enough experience you might be able to get onto an acing or extras agencies book.