What to do with "unbankable" foreign currency (UK)?

In my line of work I deal with travellers going to obscure countries and they bring me back notes and coins from countries that banks will not accept. I have a tin full now, and not sure what to do with it all! Are there any companies/agencies in the UK that can take obscure currency and exchange it for sterling which we can then bank back into the organisation? By obscure I mean things like Liberian, Bangladeshi, Haitian, Sierra Leone and coins (Euros, Swiss Francs, US Dollars etc) too. I have quite the collection!