1) Usually you will study 7 'core' subjects: Contract, Tort, Public (Constitutional and Administrative), EU, Land, Equity and Trusts and Criminal. These are necessary to get Qualifying Law Degree. There's often some sort of research skills module as well in First year. Some law schools have more compulsory modules, for example Oxford insist on Jurisprudence and Roman Law.
There are also optional modules in areas where the school's researchers have an interest. These will usually include the likes of Family, Commercial, Employment etc. but can also include some more unusual options, for example Warwick has 'Shakespeare and the Law'. These will all be listed in prospectuses and on websites to give you a flavour of what's available so see if anywhere has a range that takes your fancy.
Finally in 3rd year you can usually do a dissertation on a topic of your choosing.
2) The workload is hard, but not as brutal as is sometimes made out. Law is often quite exam heavy so it can get stressful around this time of year, whilst most other subjects have more weight on things like essays which you do throughout the year. You'll probably work a bit harder than most of your mates, but there's still plenty of time to enjoy uni life so long as you organise your time well.
3) Law is heavily oversubscribed and a lot of top unis, probably well in excess of 20, ask for AAA or, fewer, A*AA at A level in order to get on the course. Some also ask for the LNAT test to distinguish between lots of AAA candidates and Cambridge set their own admissions test. The grades are driven up by demand really, rather than only AAA students being able to cope. Certainly I don't think my course is any harder than my friends who do subjects like Chemistry and Physics which have some of the lowest entry requirements at the university. It will probably be harder to get into a 'top' uni for law than engineering but once there I think both will be equally difficult to study.
Also, you can become a lawyer with an engineering degree, in fact in patent law it's actively desirable.