If you want to study Mechanical Engineering at university, I believe you need to have Level 3 qualifications in Maths and Physics as a minimum.
Mechanical Engineering is considered to be one of the toughest degree courses so requirements are usually quite high. Most people I know who are doing Mechanical Engineering at uni have A levels in Maths, Further Maths and Physics.
In order to study those A Levels, you will need high grades in GCSE Maths and Physics. Have a look at this link, I found some useful information about studying Mechanical Engineering.