Royal Society International Exchanges: Schemes listed by country (click "Country Index") for visits to the UK (of or UK scientists to your country).
(click on Research Support, then Funding Opportunities then
(i) in Mathematics and in Theoretial Physics:
2 years for EU citizens who have/will have PhD and at most 3 years'
postdoctoral experience and are not already in a permanent academic post.
(ii) Advanced Fellowships, which are for 3 years and are not restricted
to EU citizens.
Also EPSRC accept applications for research grants from researchers at UK universities which can include requests for funding for a postdoctoral research assistant, who can be named in the proposal, but my current personal perspective on these is that the work for me to write the proposal, get the postdoc to do the proposed research rather than what they (quite reasonably) decide they are more interested in at the time, and write the final report, usually outweighs the benefits to me. EPSRC also accept applications from UK researchers to host Visiting Fellows and these are less trouble.
EC Marie Curie Individual Fellowships: 2 years for EU citizens (or 5 years residence) not a national of host country nor worked there for more than a certain time.
Leverhulme fellowships and exchanges
British Council: various opportunities depending on your country
London Mathematical Society: small grants for visits (select "Grants")
Royal Commission of 1851 Fellowships: 2 years for citizens of UK, Commonwealth, Ireland or Pakistan, about to or recently obtained a PhD.