The following guidance regarding benefits received by a UK employee from a non-resident employer confirms that, the residence of the employer has no effect on whether benefits and expenses within the rules in the benefits code are chargeable.
EIM20507 - The benefits code: expenses and benefits from non- resident employers
The benefit should be declared on the employment page of your Self Assessment return. The value of the benefit would be any amounts the employer has paid in way of medical costs and expenses which the individual would otherwise have paid himself.
With regards to the second part of your query, this would depend on whether your overseas pension contributions qualify for UK tax relief. You can fimd more information regarding this here:
PTM111000 - International: tax relief on contributions to overseas pension schemes: contents