This is the official MOT inspection manual
Headlamp, position lamps, daytime running lamps, stop lamps, indicators, hazard warning lamps, fog lamps, reversing lamps, lighting ‘tell-tales’, trailer electrical socket, electrical wiring and battery rules and inspection for car and passenger vehicle MOT tests.
www.gov.uk
section 4.4 (1-4)
No note of any symmetry required for indicators (that I can see) so I would be confident. By the looks of the major's its only if you basically don't have an indicator or it doesn't work that you'd fail! (in rough terms lol)
like Landmannnn says, I seriously doubt they'd pick up the E-mark which is probably the bigger issue. If you see the list of things to check that they have you bet that they would rather keep moving on than hunting around a rear headlamp!
Just out of curiosity I looked at a few forums with similar queries regarding the e-mark on retro indicators. one person reckoned that not all cars require E marks nor do you have to have E marked lights except on the foglight or gas discharge headlamps -
www.opsi.gov.uk/si/si1989/Uksi_19891796_en_1.htm although I can't verify that from the link.
Another interesting article here which may shed some light.
Aftermarket LED Bulbs: MOT Rules And Legalities -
The Road Vehicles Lighting Regulations states a bulb needs to be E marked, approved or have the correct wattage in order to be road legal. However this is not a requirement that is checked by an MOT testing station.
The only amendment to this is that as of January 2021, the MOT Inspection manual has been updated to include LED bulbs.
Section 4.1.4 now states the following:
“Existing halogen headlamp units should not be converted to be used with high intensity discharge (HID) or light emitting diode (LED) bulbs. If such a conversion has been done, you must fail the headlamp.”
However, this doesn't mention brake lights, tail lights, indicators or reversing lights.
Edit - notice that you've already passed an MOT in this previously with those units fitted in which case I'd say your good to go

as someone previously commented, lack of reflectors is probably the biggest deal if nothing else!