ARN-HEL-LHR means you will have to clear the border at HEL. If you have an EU passport you should be fine, if not, the staffed control at HEL can be VERY slow and I often think I don't envy the people standing in it. A friend who has a UK passport and now can't use the e-gates says of all the EU entry points he now has to use, HEL is by far the worst both in and out.