Where can I apply for a third party Travel Grant?
Speak to the conference organizers, although you claim that they don't offer travel, you don't say how you know this. Have you asked them? If you approach them with a polite email expressing your intense interest and disappointment that you cannot attend, maybe some money can be found.
Can the conference sponsor help? Often these conferences are sponsored by a large company looking to hire fresh talent. This is a good way to get noticed, so maybe they can throw a few £100 in your direction. Make sure you talk to the right person in the company.
Can your (past) institution in Sri Lanka help? Don't just look at what your department offers, scour the websites for any kind of money that is available for student travel. Speak to your former advisor. Speak to the head of the department. Speak to the careers advice team.
Are you a member of any learned society or institute? They will often have discounted rates for students and unemployed, but offer bursaries for (recent) students attending conferences.
Does your government fund any programmes that might be of use?
Are you in a minority or in some other group that gets targeted for extra money? Don't be shy, go looking for it.


Finally, what kind of price quotes are you getting for your airfare?