[Symbiote]

The security scanners were always there for the Channel Tunnel, though the check is much less intensive than for air travel. They are looking for bombs, gas canisters, guns and maybe large knives.

London had terrorist attacks from the IRA long before 9/11, including attacks on the transport network. There were 13 in 1991, of which 4 were on trains or at stations.

The new delays are for the passport checks. What used to be ~10 seconds for each EU citizen — is the passport/ID card valid and does the face match? — is now 60+ seconds for British people entering the EU, as the official must check they haven't stayed more than 90 days in the last 180 etc.