Quite simple, the Transition Altitude is what the controller tells you it is, full stop
You cannot have the slightest idea what is going on at the controllers desk, so first - take what they give you! If there are F16s scud running FL05 you cannot imagine. You may question twice, what you did, and that if fine, but they are the authority. Even further, if they give you 5.000ft on QNH 1020 and your local pressure level is more 950 - go for the 1020 and ask for confirmation, but do not think on your own with respect to such.
TA is difficult in Europe and often also related to weather conditions, as you can easily get from listening to ATIS of several stations. When under control you may even get a different TA and QNH dependent whether you fly local or x-country, so don't even rely on listening to what others got! Take you setting from traffic control.