Well happy days (island-no splashbacks to worry about). Take it you've had no joy getting the original fitters back to sort their boo boo (which would be the best way- that worktop assuming it is stone is going to weigh a lot & require very careful handling- probably cost more than my car
). Also be aware that the floor screed may not be spot level- 1mm per metre runout isn't uncommon which doesn't sound much but a 2mm gap may shout at you depending on where it is.
The plumbing may cause issues- the taps are probably on flexi tails so moving by a couple of millimetres won't be an issue. The drainage is more likely to be rigid plastic- you may need to disconnect before you relevel. The sockets on the end panels aren't a problem but may need attention (if the cutouts are tight on the cabling)
First, get your level on the worktop- it may be that the worktop is level & the units are ****ed, or it may be the other way round. Check floor level to see if there's any runout. Do you have adjustable legs on the units- if not then this is going to be very tedious. If that glue is silicon then it won't be carrying any load- cut it out (sharp breadknife).
Assuming you have adjustable legs.....check floor level. If the floor is not level (say plus or minus 2mm per metre) then decide whether you can live with that much out of level on the worktop. If you can then easy life, you need to set the unit heights so that the end panels are snug under worktop/to floor. If you can't (and worst case is the end facing you is highest) then youneed to set unit heights so the conspicuous end panel is snug as above and use a router to trim the other end panel down to size.
Re the original fitters- did you book a company to fit the kitchen or did you book one bunch to fit units & the other bunch to fit worktop? Have you paid any of them yet? You'll have paid a good chunk of cash either way, you really shouldn't be having to spend time and effort correcting their shoddy workmanship.