In UK you cannot fly without TCAS for example
That's not true. See below from UK AIP ENR:
3.5.4 Operation of Aircraft When ACAS II is Unserviceable
3.5.4.1 The current TCAS II Minimum Equipment List permits TCAS II equipped aircraft to operate for up to 10 days with the equipment out of service. This position will be kept under review.
3.5.4.2 Due to the safety benefits arising from TCAS operations and the collaborative way in which it arrives at collision avoidance solutions any aeroplane with an unserviceable transponder as well as an unserviceable TCAS will not be permitted in UK airspace for which mandatory carriage of a transponder is required
You are prohibited from flying in UK airspace if you have
both transponder
and TCAS U/S. I imagine not many countries will allow airliners to fly IFR without transponder anyway, which is why most of them have 2 installed to begin with.