Once a burglar is inside your house, he can work unseen and unheard by passers-by, so internal locks will only add a little delay. It may be useful if you hear the noise or an alarm goes off before he has time to rifle the house. This may give reassurance if you are alone in the house as you may wake up.
It's only worthwhile if you have a sturdy door. I used to have an Edwardian house with panelled internal doors 1 7/8" thick, where I relied on mortice rackbolts top and bottom (they still had the original horizontal sashlocks, which are not very strong and have only nominal keeps). Rackbolts are cheap to buy and easier to fit than mortice locks. Most modern doors, especially hollow ones, serve no purpose except to keep draughts out.
My current house has fire doors that are 44mm thick and very solid, I use a 5-lever mortice on the downstairs rooms which might inconvenience an intruder somewhat, while the alarm goes off or I hear the noise.
BS mortice locks are readily available and have a box keep or staple, which is sturdier than you will find on a cheap bathroom lock. The current BS has a long throw on the bolt. You can often get 5-lever locks which don't meet the BS but are of similar construction apart from the extended (E) bolt, cheaper. It is unlikely that a burglar in an ordinary house will bother trying to pick a 5-lever lock. Eurocylinders are not as good. and are easily defeated by low-skill burglars.
The red-cased Union BS mortice deadlock and sashlock came out very well in the Which tests, and are a very reasonable price.
Make sure that emergency exit is easy to use. You have probably heard of firemen coming across heaps of bodies on the wrong side of a locked door.
I have the back door, downstairs doors and garage door suited to the same key, so you only need to carry one, which is much more convenient that a jangling bunch. You must keep the key out of reach of an intruder.
