I did supply about 5 years ago, between jobs, and from memory it was £140 per day, £70 for a half day. I was an experienced teacher though, I think they did have a different pay scale depending on your experience so that probably sounds about right for an ECT I'm afraid. It's not good pay, but then teaching is not a career which is paid well for the job you actually do I'm afraid. It's definitely worth trying to secure a permenent position somewhere, for stability.
Marking is an expectation of the job, supply or not. Nobody else is going to mark it for you. If someone covered me for the day and left work unmarked I'd be annoyed. Often supply will just tick and initial but at least it's acknowledged. That's not a big ask I don't think.
Planning depends on the role. I wouldn't expect to have to plan for a one off day of supply (although it is worth having a bank of one off lessons pre-prepared, suitable for different year groups, just in case you turn up somewhere and plans aren't available for some reason. Thongs like a maths investigation, a science experiment, a poetry task etc. Things that don't need to link to their current learning but will see you through the day if you need to call on them, in addition to other obvious fillers such as singing, reading etc. I only had to do this twice but it's good practice to have those one off lessons up your sleeve).
If you're booked in for long term cover at the same school (eg sickness or maternity leave) you would definitely be expected to plan, but most schools would pay you at your MPS rate rather than the supply agency rate to reflect this if you're working for them long-term. Planning is often expected too if you cover in the same school each week for the same sessions (eg PPA cover). If you always delivered RE and music, for example, to the same class every week it would not be unreasonable for them to ask you to plan this.
It's definitely worth asking in The Staffroom group for better responses though. It's been quite some time since I experienced supply.