ABTAs advice regarding package holidays to Bangkok is very clear http://www.abta.com/consumer-services/travel_information/thailand and the operator is responsible (assuming it is a UK ABTA member) but if you have booked flight only through the agent you may be in trouble.

If the airline won't allow you to cancel then it will be down to insurance and you may not be covered. My policy (from Saga) does seem to cover this in their special section for Independent Travellers which accepts FCO travel advice as valid grounds to cancel or abandon. But all policies are different and many don't bother about non-package holidays so you need to read yours carefully.

So you leave London on the 27th? You can get a full refund.

http://www.omanair.com/wy/commercial_policy_BKK.html

Civil unrest continues in and around Bangkok , Thailand and several governments have issued warnings to travellers that only essential travel should be carried out. As a result we have reviewed our commercial policy for travel to Bangkok until 18th May 2010.
Commercial Policy

* Customers holding a valid booking for travel on Oman Air to Bangkok but have not yet commenced their journey
- Customers booked on flights Friday 23rd April 2010 until 27th May 2010 may rebook on another Oman Air flight to the same destination for flights departing until 30th June 2010 in the same booking class without any additional collection for Oman Air flights.
- If the ticket validity is expired, one month extension will be provided free of charge.
- Alternatively tickets booked to travel until 27th May 2010 can be refunded to original form of payment.
- All other fare conditions apply.
* Customers holding an Oman air ticket who are part way through their journey
- May rebook onto the next available Oman Air flight.
* Customers who made a booking after Friday 23rd April to Bangkok
- Normal fare rule Apply.