The answer to this really depends on you. How much you need this job?
some flexibility is necessary and from time to time the employee may
be expected to work additional hours without compensation (overtime or
time off in lieu). That these could include working outside normal
hours and on weekends and public holiday, and that they may be
required to work more than 48 hours per week.
This seems bulk standard to be honest. And most of the times they can get away with it for several reasons. Yes you are correct that you can only work 48 hours a week unless you opt-out of this (I work 100 h a week).
However remember this small caveat:
You can’t work more than 48 hours a week on average - normally
averaged over 17 weeks. This law is sometimes called the ‘working
time directive’ or ‘working time regulations’.
The WTD (working time directive) stipulates that on average if you don't opt out you can only work 48 hours a week. If they have a project and the last month they ask you to work 81.5hours a week they would still be within the boundaries of WTD (it would work out at just under 48 hours per week average). And read it again, NORMALLY, the company can decide to average it out on a yearly basis, on a 6 monthly basis, as long as they have their guidelines and they can justify it on the tribunal, the time it is averaged on can be a lot longer than 17 weeks.
Asking them won't bring anything, they can say whatever they want, until you start working you won't know what is going to happen to you.
If you need the job or the experience, then thread lightly, it might be just "contract words to cover themselves up", it can be that they copied the contract off another company (it happens with smaller companies that don't want to spend money on a lawyer going over their documents).