There are no direct flights from the UK. You have to change planes in either Barbados (BGI) or Trinidad (POS).

The flying time from London (LGW) to BGI is eight and a half hours, while the flying time from LGW to POS is ten and a half hours (including one stop in either Antigua {ANU}, St Lucia {UVF} or BGI).

From POS to Guyana (GEO) is about 55 minutes, whereas from BGI to GEO is about two hours. The POS to GEO flight would have to be on Caribbean Airlines and usually involves a long layover in POS. The BGI to GEO flight would be on LIAT and the layover in BGI isn't as long. On the other hand, LIAT is worse than Caribbean Airlines when it comes to losing luggage.

However you do it, it's a very long day. I would recommend overnighting in Barbados or Trinidad and doing the last leg of the journey on the following day.

I have no experience of working in Guyana, but I do know that it's a very bureaucratic country.