You're 5'3" and 17 - the only agencies likely to be interested in you will be dodgy outfits trying to sell you a subscription to their website/casting service or a 'portfolio' of poorly shot photos.
Here's a list of reputable
UK agencies who won't rip you off:-
However, none of them will be interested in you due to your height. Sorry.
When you turn 18, if
you're willing to take your top off then you might
have some luck with a glamour agency like Girl Management or Samantha Bond, but they generally prefer bustier girls.
Failing that, you could probably make some money as an independent model shooting art or glamour nudes. At 5'3" it's extremely
unlikely that anybody will ever pay you for anything related to fashion.
I know it sounds harsh, but that's just the way it is. Unless you're comfortable with the idea of being a nude model when you turn 18 then you need to accept NOW that modelling will only ever be a fun hobby for you. If you treat it as that and work hard to get TF shoots with better photographers then you will be able to have a lot of fun and, hopefully, get some nice images for your own use.
Your best assets in terms of making money from modelling are a pretty face and a nice figure, but you'll have to get naked - a lot - to make anything more than pocket change. As an example, here are a few well-known art-nude models in the UK.
For further reading, here's an excellent post by Madame Bink
, one of the best known and most successful art-nude models in the UK. She's been modelling (mostly nude) for 5 years and has plenty of good advice for new models thinking of starting nude work:-
Starting Modelling: Staying Safe - by Madame Bink
Just my $0.02