Hellmans has recently launched in big supermarkets over here in little 250 ml squeezy bottles as opposed to jars. I bought a couple of different ones a plain & a garlic cflavoured but AFAIK they don't do the light yet. The jars are available in the likes of Citymarket & Stockmann, the latter being the only place I've seen light version.
There is not a huge price difference - perhaps double. If you are moving to Helsinki don't bother with mayo, it's quite heavy and eats luggage allowance - better to use the weight allowance on tea.
PG Tips is available in Stockmann & Asian shops, but they sell them at crazy prices €22-24 for a box of 240. Early Grey is widely drunk here boxes of 25 are around €2.5. Boxes of 50 slightly cheaper €4.5? Bring as much tea as you can drink, the local prices are 5-10x higher than in the UK.
The local equivalent of
Dairy Milk is Fazer Blue, it's edible - a bit crisper & less creamy than Dairy Milk, but actually not that tempting to buy. So I don't.
There are no Cadbury's chocolate products on sale outside Behnfords & the ethnic shops, so we occasionally bring things like Crunchies, Frys Turkish Delight, Fry's chocolate cream, Flakes.
Pick and Mix is widely available but the tastes are quite alien and the quality is a bit iffy ... no sherbert lemons, no everton mints, no chocolate limes, no aniseed balls but a huge variety of salty licquorice and gelatin based chewy sweets - all vile. You can get toffee and strawberry bon-bons but they were never my favourite. My missus is Finnish and she prefers the selection you get in the UK too, so it's not just me.
Marmite is only AFAIK available in Behnfords, in the smallest jar size, at €5. Which is only a little cheaper than half a kilo in the UK. (4x the price)
Bring teabags, sweets & marmite. Forget the mayo.
Other food / junk food I bring: crumpets, vegetable Oxo, smoked paprika, garam masala, salt & vinegar Kettle chips, nestle evap. there are no real local equivalents.