basics:
import licence
Export licence for the country visited
vet certificates ( country of origin)
Health cert ditto ( may be combined)
Collection permit ( most places you are not even allowed to "have a look" without one)
Specific permission to export what you catch (not always a given)
A good knowledge of CITES and nationally restricted species ( long jail term for breach, if you break a local reg you may get UK gov. support, if cites forget it you are going to jail in a 3rd world country)
someone prepared to take you to places where commercially viable fishing takes place ( probably the hardest to find if you go it alone)
Permission to go visit certain areas..local and national permits may be required
Additional
Shipping agent, export
shipping agent , import
UK vets /defra health inspection
Most of the first are fairly cheap
probably £200-400 in total
The second lot
say £200-400 for export agent & £400 for import & defra
shipping fees if less than 100kg about $6-10 a kg
VAT on all the above and value of goods and freight costs. ( ie the total cost/value)
all of a sudden it looks like a minefield, because it is. IMO get someone who does it for a living to arrange it for you & deal with the paperwork, it will still cost a bomb though.