Ok, so first of all it will cost thousands of pounds.
An average 1 hour group session will be about £30 per person per lesson.
A year would get you to a very very basic level - riding a riding school pony is a totally different kettle of fish to being considered a competent rider able to befriend someone and work for rides.
Anyone who is willing to let you loose on their horse please PLEASE avoid - someone who would do that has ZERO knowledge themselves and you WILL end up getting hurt.
Literally no decent horse owner would let a complete novice have lessons on their horse with a cheap budget price instructor (which at £10 an hour is what they will be - unqualified and uninsured).
Horses are dangerous and people (in general) are stupid - decent horse owners love their animals and are very aware of the risks.
I'd be happy to let someone very keen and a total novice come to my home and help with my horses - but under no circumstances would they EVER be left unsupervised.
Riding is a tiny tiny part of horses. And the mechanics of controlling the horse are a minute parting of actually riding.
When my husband showed an interest in taking it up i gave him lessons - and he spent 3 months just in walk learning about what riders call "feel". This is the subtle interactions between horse and rider and how you can influence the horse using body weight and positioning.
Unfortunately riding schools are about getting you going as quickly as possible, so the basics are missed out and people just learn to kick and steer. Very dedicated people can turn into reasonable riders after 5 years of riding school lessons - but invariably get a massive shock when they then ride a non-riding school horse as they are a totally different kettle of fish.
Volunteering at an RDA centre, with World Horse Welfare, Redwings, Bransby, Horse Trust etc are all good starting points - be prepared to shovel shit, fill haynets, sweep (a lot), poo-pick fields... then you might be allowed to lead the horses out to fields, change rugs, groom them. Ask questions. Read stuff on line.
Once you have some basic horse handling and care knowledge then start lessons - research the riding school - the cheapest one is unlikely to be much good - you will get far more out of quality tuition in small groups that costs £30/hour than a disinterested instructor teaching 15 people that costs £10/hour.
Enquire about local riding clubs - join as a non-mounted member and volunteer at competitions and training nights. You'll learn a lot watching people and what's going on. Make friends, ask questions.
If you are lucky, you will then find a caring and conscientious owner that is willing to exchange knowledge / the chance to interact with their horses in return for help with chores.