Stuchbury - I have to disagree with eggy. Each year I've lost satellites in the long tunnel and it's switched to footpod. If you don't get one add a manual split at the next mile marker and you'll be back on track with pace just overall distance will be less.
Stuchbury - if you're happy with miles go with it. There are mile markers but not as visible as km markers. Have you got a footpod for the tunnels? If so go for your garmin. Garmin will measure long though so you need to pace about 5-7 seconds per mile faster than exact race pace. Hope that makes sense.