How to change agents
So it would not be a good idea to your agent at the moment (assuming you do have one) that you want to change agents as they may not take this too well, as not without reason contact may start to become lessened and the auditions they get you may now start to slip away. You want to get a new agent - that's your business. Lets start looking.
Grab a copy of the most up to date version of the directory Contacts, which is published by Spotlight. You can get it directly from Spotlight, or from hundreds of places online, actual real-life bookshops (remember them?) and there sometimes are free copies at your library in the reference section; usually in the larger libraries. In it will be a section on agents which lists every agent in the UK that chooses to be in Contacts - London, around the country, the co-ops, the top ones and all the rest of them. Go through the list very carefully and thoroughly, and select the ones you want to get in contact with, and write to them accordingly, via email or normal post depending on what they request, including everything they need, and leaving out everything they don't need (they are busy people, bless ‘em). Do follow the rules they state - don't think that if they request that you write them a letter and you email them that you will stand out, you will merely take up about half a second of their time in which they move the mouse over the ‘delete' icon, and click on it. Another good directory is ‘The Actor's Handbook', also available everywhere, which includes some information taken from agent's websites about how best to write to the them, although it's always best to check directly anyway. Remember, every detail of this can be very important work in that you may get a new and/or potentially good agent from it, therefore you should take time on this process to do everything properly; you don't want rush it at all; you've got the time.
If you don't get a new agent from the letter/email writing, it's your choice to stick with your old one or not. You may still be getting auditions so it may still be worthwhile staying, but if you are not there is a reasonable argument to be made for leaving. You will then be an unknown, and you can still apply for castings on Spotlight, Casting Call Pro and the other services out there, but now you don't have the maybe not very good reputation of your previous agent holding you back. You become less able to be identified in that negative way, and so to casting directors and other people who may hire you, you could be an undiscovered talent, rather than an actor from a not very good or well reputable agency.
So write the letters or emails, and take make sure the address and tailor each letter to each individual agency. Mention the actors who are there who you have seen on stage or screen, mention a friend who is with that agency who has recommended them, mention that you have heard that the agency or agent has a great reputation, anything to get the letter to being specific and more personal, rather than a generic bland letter. There is nothing wrong with talking about your current agent in your letter, but it is bad form the slag them off. Instead, be constructive about the change you want to make, for example you have been with them for a long time and you fancy a change, or that you are mainly getting commercial castings and you have enjoyed that work, but you would like more opportunities for stage work, and your agency has actors working across Britain and the West End in some very high quality plays, that sort of thing.
And so keep writing and writing. The vast majority of replies (if any) you get will be negative, so you keep going until you get a positive one and maybe a meeting, or an acceptance. Persistence is the key with this, and if you don't go with Independent Talent or Conway Van Gelder Grant this time, they will remember you next time you want to change agents and they see all the brilliant work you've done since the last time you wrote to them. So maybe this next time they'll be calling you in for a meeting. So keep going, and good luck.