What makes a tomato "heirloom" and what is the difference to conventional tomatoes?
Edited: heritage -> heirloom, makes more sense that way.
In the US, I normally see them marketed as "heirloom" tomatoes; I don't know if there are specific regulations in the UK or Finland that might have restrictions on the term's use. (like there now is in the US with "organic" )
Because they weren't developed for industrialized farming, they didn't focus on breeding in traits that were desired for mass production -- size, round shape, bright color, shelf life, durability in transport.
As such, there's a much wider variety, and they tend to be locally grown on small farms (as they don't tend to transport well, or be suitable for mechanized production). They might've been bred for sweetness, tartness, number of fruit per plant, disease resistance, etc, so you can't say that heirlooms necessarily taste better; a locally grown, picked that day non-heirloom tomato might taste just as good.
To make this clear -- very few vegetables are produced in a truly 'uncontrolled' environment (maybe 'less controlled' environment)-- humans have been selecting for specific traits for centuries; it's just that the processes used were much less high-tech back then -- you saved the seeds from plants that produced what you liked, it's possible that some might've even been hand-pollinated to cross breed cultivars.)
A heritage tomato is a strain of tomato that is relatively unchanged since it's original discovery and use. The originals traits are maintained by using open pollination via insects and what not in an uncontrolled environment. Selection techniques such as grafting and cutting (to clone and create hybrids), or flat out engineering are not used, while the lineage of the plants must date back at least 50+ years.
So a heritage plant will be pretty much something that would see before industrial farming came to existence. There are a lot of varieties, some of which are relatively rare and exotic, but it doesn't really tell you anything about the quality or taste.
I guess you can be assured that the plants have been grown at a somewhat smaller scale than large industrial farms, and that the plants haven't been genetically engineered, but that's not necessarily a positive to me.
A heritage tomato is one that has been passed down by seed generation after generation. Most grocery store tomatoes are F-1 hybrids developed for qualities such as disease resistance, storage, etc rather than for taste. You can't save their seeds and grow them as the seeds will either be sterile or revert to one of the parents. You can save the seeds of heirloom plants & grow the same plant from them. They have the old-fashioned tomato taste, so they are preferred by most people who really like tomatoes.