Why does a USA/Canada surge-protected extension lead (110V) trip my circuit breakers in the UK (220V)?
The surge suppression in most consumer grade power strips includes a metal oxide varistor (MOV) placed across the line at the supply side of the power strip:



These devices suppress voltage spikes by conducting energy across the line when the incoming voltage exceeds a preset threshold determined by the manufacturer.  Wikipedia has a good description of varistors here:

https://en.wikipedia.org/wiki/Varistor

MOVs in a surge suppressor designed for 120v are selected to conduct at roughly 200v.  This ensures the MOVs do not interfere with normal operating conditions, which include peak voltages up to 55% greater than the nominal line voltage.  If you connect one of these across 220VRMS where the nominal peak voltage is 310v, far into the conduction region of the device, then the MOV conducts during normal operating conditions shorting out your extension lead.