Given that the Air Travel Trust has £170 million of cash and £400 million of insurance; that when Monarch Airlines failed they could repatriate "around 110,000 passengers" for £16.3 million [1]; and that Thomas Cook's collapse leaves about 150,000 holidaymakers [2] in need of repatriation; it seems unlikely the funds will fail to cover the problem.
There might be other expenses, outside the scope of the ATT, that the government ends up paying - there are estimates as high as £600 million [3] - but arguably that's a sign the scope of the compulsory insurance scheme should be broader, not narrower :)
[1] http://publicapps.caa.co.uk/docs/33/ATT_Accounts_2018.pdf
[2] https://www.bbc.co.uk/news/business-49791249
[3] https://www.bbc.co.uk/news/business-49781920