...You can always tell if it's a 3 or 5 levers lock by looking at the key, they either have 3 or 5 cut out on the key.
more than that because they have to operate from both sides. Just looked at mine (5-detainer), it has 9 bits (the middle one is used from both sides) so from one side it uses bits 1-2-3-4-5- and from the other 5-6-7-8-9
My back door is 5-lever but key has 7 bits (some of the levers are the same)
Even a safe lock (operated from one side only) has at least one bit more than levers, the extra one operates the bolt throw.
I agree about the
Wickes own-brand, they look good value
The popular Chubb BS locks are part no's as salem2000 says, but they now have an "E" suffix for the extended (20mm) bolt of the newest standard.
If buying a BS lock make sure it is stamped BS3621 - 2004 (is better than the older standard, but some old stock is still around - especially on Ebay where they may look cheap)
There is a range of BS locks on
http://www.screwfix.com/cats/A357887/Security/Mortice-Locks/BS-5-Lever-Deadlocksl
and
http://www.screwfix.com/cats/A359606/Security/Mortice-Locks/BS-5-Lever-Sashlocks
The sashlocks are usual on back doors. Front doors usually have a nightlatch as well (Yale type) which is not much good but convenient
If you can only afford one BS lock put it on your Final Exit door, and supplement the locks on the other doors with rack bolts.
However a 3-lever lock is usually intended for a bathrom or bedroom door and is not intended to be secure - feel the weight of it and see how weak it is.
Hide or lock away the keys - never leave them in locks. if you are worried about exit in an emergency you can put a cup-hook for the keys at the top of the door frame, just within reach of the smallest person in the house
p.s.
NP and EB type
these are both cheap options. NP is nickel plate (white metal) EB is Electro Brassed (will go rusty)