Leiden University publishes a list of diplomas that should be considered sufficient to meet the minimal requirements. If you want at least some certainty, then you do need to get a diploma that's on it but even that list comes with a lot of caveats.
Applications from prospective students with other qualifications will presumably be evaluated on a case-by-case basis, which is probably why you won't find anybody willing to make any promise beforehand. At the end of the day, individual decisions will be made by each department based on the full application and whoever you talked to probably does not know all their criteria and can't realistically commit in any way beyond what's already published.
For what it's worth, here is what's on the list regarding the UK:
- At least 3 GCE A-levels in general, academic subjects with grades A-C
Please note that vocational qualifications, even though taken at the same National Qualifications Framework level 3, are not considered sufficient.
- Access to Higher Education Diploma or Foundation Diploma for university admission with good results (A-B) in relevant subjects
- Scottish Qualification Certificate with at least 4 Scottish (Advanced) Highers in general, academic subjects with grades A-B for Higher and A-C for Advanced Higher subjects.
Do note that it's “3 GCE A-levels in general, academic subjects”. Another webpage provides more detail on this (it's only for the Bachelor of Psychology but it probably applies to other departments too):
Please note that most BTEC A-levels are not regarded as academic A-levels. By academic we mean science-related, for example maths, biology, geography, etc. Design, photograpy, art, etc. are not regarded as academic A-levels.