Is 100 hours per year of teaching a heavy load for a lecturer?
The load seems reasonable for an established lecturer, but is a little heavy, by UK standards, for a first year lecturer. It might be worth asking for a partial teaching (or marking or tutoring) release for the first year.

Many universities, including those in the UK and US, talk about the split between research, teaching and admin. A split of time of 40% research, 40% teaching, and 20% service is not atypical at a UK Russel Group university. In the UK, the work year consists of about 1800 hours (37.5 hours per week times 48 weeks a year). With a 40% teaching load, you should be doing 720 hours of "teaching". I have never heard of a Russel Group university with a teaching load lower than 30%. The non-Russel Group universities I am familiar with don't go above 60% teaching time.

Teaching obviously consists of more than just standing in front of students lecturing. My UK university work load model credited us with time for supervising undergraduate final year project students, our mandated office hours, marking, tutorials, and lecturing.

We typically had 40 office hours (2 hours per week of the two 10 week semesters) and 150 grading hours. This left about 530 hours of traditional teaching time (tutorials, lectures, and practicals). For every hour of tutorial and lecture, we are given either 4 or 8 hours of prep time depending on if it is new teaching or not. This means an established teacher would have 132.5 hours of lecturing a year and a new lecturer would have 66.25 hours of lecturing a year.