Thorpe Park have in house security, they have recruitment days on 2nd and 3rd Feb. I believe most theme parks in UK have their own in house. I have put the link for the pre registration form for Thorpe park, also the number if you want to ring them. They have CCTV operatives and from what I can make out a security team who operate within the park, along side the CCTV operatives. Hope this is of help to you.
HR - Thorpe Park is 087044446783 Merlin Enterainments Group - Serious about fun