My new contract is stating that the maximum weekly working time in the Working Time Regulations will not apply to me.
This may simply be a true statement, rather than an attempt to opt you out of the directive in the contract (in that it is a job excluded from the rule by the Working Time Directive, e.g. armed forces). There is general information on this from gov.uk. There are also occupations where you cannot opt out and your time is subject to other regulations, often for the safety of others (e.g. heavy goods vehicle drivers).
If you are being offered a new contract and being asked to opt out at the same time, you are more likely to be in a strong place bargaining-wise. Remember, you can refuse now and keep your current job: once you accept, there are restrictions on when you can do so (the TUC's Worksmart service has good advice on this (pdf)).
If you otherwise have no problem with the job offer, you could return the contract with the relevant part struck through: that would force them to either accept the change or tell you it's mandatory (in which case you have either their reason, or else you can keep working your existing contract). It may be a sign they have a long-hours culture, but might just that they want to avoid responsibility. If it's an office job, 40 hours is long to begin with for the UK, 35-38 is more typical. If you know and trust someone who already works there (not the hiring manager or HR contact), you could ask them to see if long hours are ever an issue.
If they want to keep the clause and the job offer is still tempting, asking about their overtime arrangements may help you decide - e.g. would you work 48h+ if they paid 'double time' for the additional hours?