Since no one else has responded;
If you're talking wet lease of more than five business days, you need to speak to all other possible UK sub. operators for the charter boys such as EAL, Channex, Titan, etc. to see if they have aircraft available to service what you propose. If they don't, then you may perhaps "import" a tu154.
Is Hemus a JAR operator? Is Bulgaria a JAA member state? If not, you will also need to speak to the UK CAA and pay them lots of money to check if they meet the equivalent safety standards of all UK based AOC operations.
Maintenance? Well, I doubt if anyone in the UK would cater for a tu154, but the aircraft should come with line engineers and enough airframe hours to see you through a charter season.
Not sure on the cabin crew issue, and in any event, which UK Charter operator is going to lease in a tu154 on long term pax operation carrying British punters???
