The current position
As a Finnish and EU national, you currently have full rights to enter and work in the UK, ie to visit children’s parties and department stores and to deliver toys on Christmas Eve before flying back home after a temporary stay.
The toys you deliver can be imported from Lapland into the UK without any import duty or tariffs and will automatically meet the identical EU standards applied to children’s toys in the UK.
Your work in future will depend on the terms on which the UK leaves the EU. It seems increasingly unlikely that we will continue to have unfettered access to the EU’s single market. Whilst what will replace it is not yet decided, we know there will be a transitional phase (or implementation period) during which existing EU rules will continue to apply. The UK is hoping to conclude a new Free Trade Agreement (FTA) with the EU for the longer term to govern trade in goods and services.
What I can advise at this point is as follows:
Your rights will likely come to an end after the transitional period - the duration of which is still unclear. If no further deal is reached with the EU, you would then be subject to UK immigration rules meaning you could not work here without sponsorship or a visa allowing you to do this. Although you are well-known internationally, you would not currently meet the requirements of the UK Tier 1 (Exceptional Talent) visa as this only applies to those working in the sciences, the arts or the technology sectors.
If no future UK/EU trade deal is negotiated, the UK will apply World Trade Organisation (WTO) tariffs by default meaning you could pay up to 10% tariff duties on the value of goods imported to the UK. There may also be complex “rules of origin” issues to be considered (particularly if your toys include components made outside the EU) which would add to the duty to be levied. Despite the fact you are not selling these products, you may still be charged.
If the toys made in your workshops bear the EU safety kite mark, this will probably suffice for UK users under the Great Repeal Bill, although I cannot, of course, rule out that the UK will choose to adopt different standards in the future.
Should you wish to establish a toy making facility in the UK, you would need an entrepreneur visa, requiring an investment of at least £200,000. However the fact you would not actually sell products commercially here would mean that you would most likely not meet the “genuine entrepreneur” test. Neither would you be able to sponsor your elves to come and work in the UK as current UK immigration rules do not allow sponsorship of lower-skilled workers. You should strongly consider setting up any UK facility before Brexit and transferring your staff in before the end of the transitional period.
Hopefully by next Christmas there will be greater clarity on the UK's Brexit and future immigration plans so that I can advise you with greater certainty.