I don't know the answer to "where does it end" but Britain has certainly believed in and, at times, insisted on reparations where it was the wronged party (e.g. after WW1).
The nations that Britain harmed did not have the same leverage to demand reparations at the time slavery ended, as Britain did in relation to Germany post-war, but there have been some calls for reparations for slavery dating back centuries (to when it was abolished), so these are not new demands but rather demands that the UK has refused for centuries.
I don't think the Romans are a good example because I don't think many would argue that modern British people are economically disadvantaged by the former Roman occupation, and of course the Roman state no longer exists. People living in formerly colonies and other relevant countries do still suffer economic hardship as a result of colonialism (a lot of the direct evils of which, like stealing Indigenous children from their families, were still happening in Australia and Canada within living memory - although those governments have now made some payments of reparations).
I do think this is all academic because even if a British government had the will to pay reparations (which it wont), the country doesn't have the means - but I do think "what about the Romans", etc, is a little trite.