Listed below are the minimal security requirements of most insurers but it is always advisable to contact your insurer to confirm theirs.

Security Requirements:
Loss by theft or attempted theft from the home is not covered unless the following security measures are in operation when the property is left unattended. All external doors and windows must be closed and locked and keys must not be visible from outside. [ I’ve added the underlining ]
The final exit door….
i is fitted with a mortice deadlock with at least 5 levers; or
ii is fitted with a lock conforming to BS3621: 1998 or to a higher specification
In addition, where the final exit door is double-leaf, the first closing leaf must be fitted with top and bottom key-operated security bolts. Bolts must be positioned vertically so as to enter the framework of the door, not the opposing leaf.
Double-glazed PVC-U, Metal or timber doors
Key-operated multiple-point locking devices on double glazed PVC-U, metal or timber doors with an insurance approved cylinder lock are an acceptable alternative for the security measures detailed for doors above.
All other single external doors….
i are fitted with key-operated security bolts at the top and bottom; or
ii are fitted with a mortice deadlock with at least 5 levers; or
iii are fitted with a lock conforming to BS3621: 1998 or to a higher specification
Sliding patio doors….
i fitted with two key-operated patio door locks on the inside of the doors at the top and bottom of the frame; or
ii are fitted with a key-operated multiple locking system having at least two locking points
Windows
All opening ground floor and basement windows, and all windows which are accessible from roofs, walls, fences, balconies, landings, porches, soil pipes, drainpipes, trees or shrubs must be fitted with at least one key-operated metal lock
Alternatively, these windows are permanently fixed shut by means of non-return or defaced head screws angled through the window frame into the timber or metal surround.