Homemade jam will have a higher fruit content, and (usually) a lower sugar content - plus no additives, etc - than is usually the case with commercial jams, unless you use small, artisan, excellent (and, unfortunately, usually quite expensive) French or Italian jams - and, homemade jam will also - almost invariably - taste far better.
Both the UK and Ireland have (and yes, France, and Italy...) have excellent local, farmers. markets, - often with a pedigree going back to the Middle Ages - where people sell locally produced produce, cheeses, jams, eggs, honey, fruit, vegetables, meat and so on - the fruit and vegetable delivery last night came from a market stall that had emailed me to let me know that they were heading off on a month's holiday (for some sun) and wouldn't be around until late October; otherwise, I wouldn't have placed that order until next week. These markets will usually take place weekly, - often, a Friday or a Saturday - or, perhaps, monthly, in some very small towns.
Anyway, in a local market, you will have an excellent opportunity to purchase locally made jams, and the quality is - almost always - far superior to what you can buy commercially (apart from the high end Italian and French artisan producers, and those you will only find in small, specialist stores).
Believe me, you don't have to sell me on the quality! I'd always heard that food in the UK 'doesn't taste as good' or 'is kind of bland.'
Within about 2 days of landing there, I'd figured it out....the food simply doesn't have the GMOs and additives and preservatives and the like that a lot (but not all) of the food in the States has. Once I gave myself time to acclimatize, so to speak, I found myself liking the food FAR more, and it's not just food. I can eat candy in the States, and the same candy in the UK has far fewer ingredients, far more of them are natural, etc.
But yeah, we spent quite a bit of time just walking around, buying various jams and snacks and foods to sample, and it was absolutely fantastic. Looking forward to going back in 2024, I hope (although will probably spend far more time in Scotland than England, TBH - wife fell in love with Edinburg and Inverness).