No university is inherently strict or lenient when it comes to grades. It all comes down to the numbers. They'll have a good idea, from past experience, how many offer-holder will meet the grade requirements of their offer. They are obliged to confirm the place of all such applicants. Depending on how many students meet their offer requirements, this might mean they take-on more students then they'd planned for. Or, it might mean that they have some space places. If they do, then they'll likely look to fill those places via those candidates who just missed their offer grades. If they do, they're not being "lenient", they're simply reacting to the difference between the number of places on the course and the number of students who met their grade requirements.
Whilst this is absolutely no guarantee of what will happen this year (as it depends on the numbers - see above), in recent years 98% of those who were holding an offer on results day and achieved A*AC were accepted. That number dropped to 82% for AAC. (See the "Historical entry grades data" section of
this page on the UCAS site). Note that some of these may have been contextual candidates, or had other special circumstances which may make these statistics misleading.