Re: travelling for interview, etc..yes, there is parking at Waterside but given even existing staff temporarily visiting Waterside (e.g. for a medical) find parking there somewhat problematic due to lack of space I’d second the advice given to use public transport and/or one the frequent shuttle buses....especially if you are up against the clock...
If you really really have to drive into LHR then consider using one of the perimeter car parks and then either their own shuttle buses, or one of the TfL buses (hopefully within the free zone) to travel onwards..FWIW the weekday BA1 shuttle bus that goes via T5 and Waterside actually picks up/drops off up at Hatton Cross at one end of it’s route, and Harlington Corner at the other....just don’t forget your letter...