You may need to say where you are for people to comment. I am out of the supply game at the moment but have several close friends who do it locally (North West). I think it depends enormously on where you are - the bigger cities with more children and more schools will obviously pose less of an issue than rural areas. If you are secondary trained in a shortage area, things will probably be easier if you want long-term roles but the day to day work is now very much covered by cover supervisors and the pay is awful. You would be lucky to get beyond M6 on longer term supply, and then only after 12 weeks when AWR kicks in unless you are particularly good at negoiating or someone in SLT in the school knows you are worth the money.
It's a game and you need to play it - sign up with all the agencies you can find (you only need one DBS on the update service) and sign up now for September because it takes time for references and DBS to be done. Some schools with known September vacancies will already be using agencies to find people for them - so again, if you're not signed up now, you're starting to miss out. Agencies want good staff on their books and will sign you up with promises they are unable to keep - they are all chasing the same work at the end of the day so if you don't sign with all the agencies, you will miss it as some schools have preferred agencies they work with.
You need to allow for the agency to get to know you. It gets easier when you get good feedback and/or schools start asking for you by name. You can pull in £23k in a year but I wouldn't bank on it for the first year if you end up having to accept day to day cover supervisor work to get known a bit.
If secondary trained, you may struggle to get primary supply - probably depends on the area but there isn't the shortage of teachers in primary generally that we are all seeing in secondary. However, if you are shortage area trained, you may find a niche in primary such as MFL. If you want to switch to primary, there is any number of companies out there providing PPA cover in specialist areas - MFL, ICT, PE, music - so do have a look at those. I have no experience with special schools. I do know the local PRU and all the local sink schools struggle to get long term supply staff so if your behaviour management is good, there's probably something out there for you!
When you get in school, make it known to receptionists/cover organiszer that you've enjoyed the day and would love to come back. This helps enormously in terms of getting asked back. Tell any teacher you speak to that you love it and want to come again. Make your specialism known in schools you really like because they do remember you when stuff comes up.
Hope that helps.