yes, you must have a BS 3621: 2007 deadlock. 5-lever locks have security advantages over locks with a Eurocylinder. 2007 was the date of the most recent ugrade to the standard, and is more secure than older locks. The BS number and date are stamped on the front of locks so you can see it, with the kitemark.
A Which best buy is the red-cased Union which scored better than other locks from prestigous makers at several times the price.
there are examples of BS locks here
Wickes sell a small selection. Their own-brand ones are actually made by ERA and are OK.
As well as a deadlock, i expect you will want a nightlatch. the old-fashioned "Yale" type nightlatch is laughably insecure but is adequate to stop the door blowing open except in strong winds. You can get BS nightlatches which are bigger and more secure. Because nightlatches are intended to be opened from inside without a key, the BS ones have the facility to lock the knob so that once deadlocked, they cannot be opened by reaching through broken glass, or a hole in the door such as a letterbox using a simple home-made tool; nor can they be opened by someone who has climbed in through a window, to make it easier for him to carry away your belongings.
A nightlatch is a useful supplement to a mortice deadlock, but should not be considered a replacement.
You can choose locks and escutcheons in brass; or polished or satin chrome, nickel plate etc to suit your whim.
If you have two locks, one should be positioned a third up from the bottom of the door; and one a third down from the top. Traditionally the nightlatch is at shoulder height and the deadlock at hand height; but I now think it would be more convenient the other way round, unless you have small children and you want the knob out of their reach.
I would also recommend a sturdy doorchain by ERA or Yale. this design is sturdier than most. You may still find old stock with the Chubb brand name in a similar design.
The letter box should be positioned as far as possible from any locks, bolts or chains, especially a nightlatch, and preferably not in the door, but to the hinge side of the doorway.
You will also need three hinges, in brass or stainless to match the door furniture. I recommend lift-off hinges which cost a few pounds more but make future maintenance and repainting much much much easier.
The parts of a door which are in most critical need of protection by painting are the top and bottom. The parts of a door which are most often neglected due to being difficult to get at are the top and bottom.