I think the first thing to you need to assess is what you would like to do as a career. Chemistry and/engineering can lead down many different avenues in terms of career prospects. There is ultimately no right or wrong answer and for each individual it depends completely on how motivated you are and what you want to do in the future.
If you take solely chemistry you will get a good rounded knowledge of Chemistry and will have a broader range in that field. that's very useful to have if you want to go into post graduate study and complete chemistry to a higher level leading onto research. with a BSc in chemistry you can look at going into development work, analytical work, accountancy as many chemists use more mathematics than accountancy students so its a good opportunity. If you want you can usually complete a 2 year conversion course afterwards in Law and look at the route of becoming a patent attorney (very well paid and sought after). companies that work with raw materials will always have jobs for chemists... you've be surprised even places like Gregs Bakery have a team of chemists!
As for engineering on its own - there is a host of jobs out there in the construction, manufacturing, automotive and aerospace industries. its a case of what interests you now and what do you think will interest you in the future.
Combining the 2 opens many more doors, but also closes others. by doing a join honours degree you are putting yourself in a more niche position by having 2 skill sets, but at the same time removing yourself potentially from something that requires more of a chemistry of engineering knowledge as you will ultimately have less exposure to both subjects.
My advice would be to go ahead with whatever your heart tells you and what you think you want to do in the future. my personal experience would tell me that Chemistry is the best option, but if you want to get involved in the automotive industry I'd say Engineering - especially with central based universities and Jaguar Landrover and JCB on the doorstep you'll be very well placed to look for jobs as an engineer with potential of doing summer work. That said if you want to work in exhaust emissions after treatment the best bet is going for the combined!