In answer to Slim75’s question - it may be dependent on the ATC unit’s equipment - squawking an emergency code will flash red and highlight your aircraft data block on the screens we use, but we will still see your original details (eg callsign converted if you were on an airways squawk, or original squawk code if not).
In response to the second poster’s comments on the lack of use of an Aldis lamp during radio fail at Birmingham - the unit does not physically have one to use.. and has not had one for many years. It would be interesting to see if many modern towers have one available- my suspicion is larger modern units won’t have, unless specifically mentioned in the AIP?