It would certainly be infuriating and terribly disappointing to have the experience you had. But what happened here (as I understand it) is that as with all popular exhibitions at museums everywhere, to prevent overcrowding an estimate had to be made as to how long visitors would spend viewing the exhibit. The amount of time allowed for was an average of 40 minutes, which is almost twice as long as the average time visitors take in France. The British Museum ticketing was based on this - it is not correct to call this "overbooking". It was a reasonable and generous calculation, taking into account the experience of the museum in France. However, it turns out that visitors in London are taking much longer than in France to view the tapestry. As a condition of lending the Tapestry, there is strict limit on the amount of light it can be exposed to, which the museum must rigorously observe or risk damaging it as well as breaching the agreement. So they cannot extend viewing times or take a lax view when people take longer than expected in the way museums normally do. That is why I say it is not really the British Museum's fault that ticket holders are being turned away, although I agree it would be maddening and upsetting.