They certainly wouldn't harm your chances.
To be honest though,
all Universities you will be looking at (regarding Medicine) would generally look for extra-curricular involvements outside of the usual Volunteering/Work Experience stuff as an enhancement to an application, so that wouldn't be limited to Imperial.
Of course it's not the end of the world if this isn't extensive, but I would advise you to take some sort of extra curricular activity before you apply, just to make sure you are able to draw on these experiences in your Personal Statement and at interview.
Although that being said, in my Imperial interview, for example, they probably gave about 8 minutes on Work Experience and 4 minutes on Extra Curricular (as well as a few other bits and bobs), which was probably longer (on ECs) than either my UCL or King's interview - so from
my experience alone, I'd say Imperial weigh your ECs in fairly high regard.
Again contradicting myself, I know others that were asked about their ECs more than I was at both UCL and King's, so it really depends on the interviewers themselves, as much as the specific University requirements.
The whole process of applying is subjective, so if you don't feel you'll be able to commit to many ECs, then make sure you've got a lot of W.Ex to talk about - that sort of thing. They're after well-rounded people for sure, but they'll also love being impressed by something else on your application, and may forgive a leaner EC if another area of your application is particularly strong.
I'm waffling now; basically, give yourself every chance by being fair to yourself.
Best of luck with your results - are they on the same day as ours (next Thursday)?