Both are good for Engineering subjects. Both have excellent Eng depts with excellent facilities.
Loughborough - smaller town, smaller University so more sense of 'community'', Uni is big on team sport/sport generally. Uni is relatively modern on a self contained site - ie. housing on campus.
Southampton - much bigger Uni, much bigger city, the campus is in a S'ton suburb with public roads running through it, mixture of buildings, locations etc.
So - it depends what you want in a University. A Southampton Eng degree will not be 'worth' less than one from Loughborough.
Have a look at the Campus Tour films for each Uni on YouTube - there may also be their subject films. Think carefully about what you want from your Uni experience. There is nothing to stop you just phoning/emailing S'ton to see if they would accept you - but you would have to be 'in Clearing' for them to make you a formal offer.