What is a "normal" teaching load for a Lecturer in a UK pre-'92 University?
I can only answer for biological sciences:

Universities will have something along the lines of a "Workload Allocation Model" system for dividing up work. Under our departments accounting you get 3 hours prep time for every formal teaching session you give, plus 20 minutes for every exam script you mark. Tutorials don't count as "formal teaching sessions" and so get no prep time. 

For for example a 10 credit module, with 18 lectures,  and 2x 3 hour practicals for 50 students would get you an allocation of 100.5 hours. 

A full time contract is ~1600 hours, so a 50% teaching load would be 800 allocated hours. However: 


50% teaching load would be unusual - the normal split is 40:40:20 teaching:research:admin
There is generally an (informal) recognition that prep takes longer for new hires than for old hands. 


A common load for a lecturer would be very little in the first year, perhaps one module in the second, rising to around 30-40 contact hours a year once you pass probation (about 4 times the above, if you had the same mix of lectures, practicals and tutorials), and then rising again to a "full" load of around 120 contact hours as you become more senior/part of the furniture. 

There is obviously a massive variation in this. If your research goes really well you might never rise above the ~40 hour category, while some of us can teach things the department desperately needs, but no one else can teach, and so have to do more. 

For example, I was on a full load of 120 hours within three years of arriving (which was unusually large). My experience of this sort of load is that if you do the teaching well, then you are basically a full time teacher during term time. This makes securing the grant you'll need to pass probation difficult to say the least. While semester is in full swing I only really have time to meet each of my mentees once a week (note that supervising postgrad students counts as research not teaching) and run my lab meeting and journal club on top of my teaching and admin duty. I then concentrate on pushing my various research projects along properly during the vacations.