General Rule of thumb when choosing your university is that Overall reputation > Faculty Reputation. Employers always pay more attention to overall 'branding' than individual strength of the university's faculty. The only exception is when the course is a very specialised and professional one such as Engineering (which is your situation) and perphaps law. If i were you, i would look at the issue in this manner:
UCL's Strength:
1. London advantage - The location may sound pretty irrelevant at your current stage but you will soon feel its impact the moment you start your education. Apart from the 'social and fun' aspects, many major events and high profile international speakers come to london, etc so its really a good eye opening opportunity.
2. Stronger Overall reptuation - UCL attracts many more top employers than Manchester to hold recruitment events at their campus. The proximity to the financial centres also means easier time for networking sessoins. I m not sure how much stronger is Manchester's engineering but UCL will definitely give u and edge when u are applying for non-engineering jobs such as finance etc.
3. Many top firms only hire from a select few target schools, and this is never going to prove more true during this financial crisis. For example, in the current market, u hardly stand a chance to get into top IBS or consultancies if you dont come from oxbridge, lse, imperial, ucl, warwick etc without top-notching personal connections.
Manchester's strength:
1. A stronger faculty will probably give u better employment prospects when your application is reviewed by employers who are in the industry and who are AWARE of this particular strength of the engineering department.
2. Stronger departments boast of more high profile reserachers whose reference may of greater weight if you are applying to graduate schools. However, given UCL's general reputation i wont personally see this as making overly significant difference.
So overal i think UCL should still be your first choice. Yea, hope to see ya here next year. I also did my a levels in Singapore, btw haha