Normally if it's accredited it will state that - I don't see anything about accreditation so assume it is not. You may wish to contact them to confirm if the course is accredited or if they are in process of seeking accreditation for the course.
Usually it's straightforward to switch between the BEng and MEng versions of the course during the first two years from the internal process perspective, it's typically just an admin form to complete - this is not specific to UCL but just in general with engineering courses in the UK.
Note there may be financial or visa implications for home or international students - generally for home students, moving from the BEng to the MEng can cause some complications with SFE so it's better to start on the MEng if uncertain. Conversely for international students as I understand it's easier to extend a student visa starting on the BEng to extend to the MEng, rather than cutting one short to do move from the MEng to the BEng, so it may be simpler to start on the BEng if uncertain in that case.