If you can have your hold luggage checked through for the full journey you shouldn't need to clear passports/security at all, and even if you are changing terminal there is an airside bus between the terminals. Otherwise the free GatwickConnects service looks like a good deal because they take your luggage off you at baggage reclaim on the inbound flight, print your boarding pass for the outbound flight and you get fast-track security as well.
Passport control at the North terminal can be a nightmare, especially if you land just after one of the Emirates A380 flights from Dubai. The South terminal is rather better. Equally you can just walk through EU passport control at both terminals if it's quiet and/or they have all the e-gates open.