There are a number of items that require special criteria to be met in order to allow their customs clearance into Thailand. Foodstuffs is one of them.
It must be in a Thai Bank account which is in your name. The money must have been cleared in the account at least 2 months prior to making your application. Bangkok Bank has a branch in London and is the only Thai bank in the UK. You may find it easier to deal with them and that there are advantages having the same bank in the UK and in Thailand.
Nepal has already delayed resumption of international flights once and has not issued details of which countries' flights will be permitted or what the quarantine requirements or conditions of entry will be. However, the trekking season runs from Sep to Nov which is worth a lot of money so they will want to make the most of this income.
Governments charter repatriation flights on behalf of its own citizens of its own country. Unless you are a citizen of the country where the flight is going to then chances of getting on it a small as they fill up quickly.