As mentioned, no extra security at LHR.

No real way to shortcut immigration, without the immigration. Not usually too bad, except Fridays which get very busy at that hour.

On departure, there is a business/first "line" at the security check area. Just circumvent the check-in area towards the end of the hall and you will see a sign for first/business class passengers. Just stand by the sign and one of the security folk should see you and come to you. This can save alot of time, but won't change the questioning procedure.

last 3 check-in desks on the right are Club, First and Cust service desks and also don't tend to have long lines.

I would leave 3 hrs though, to be on the safe side.

Shopping has lots of options, but nothing very cheap.

Have fun, weather should be good (despite a nasty storm this week) ^