TWENTY-EIGHT expatriate civil servants are sailing home on board the luxury liner Canberra on March 4 despite uncertainty over the future of the retirement perk. The year's take-up rate is the highest for the past five years. In 1989 and 1990, the tally was 15. The number rose to 17 in 1991 and to 23 last year. The total cost of the trip this year is estimated at $3.4 million, with the 28 officers being joined by 29 immediate family members. Although the Government remained tight-lipped on who would join the cruise this year, it is understood that among the big names taking up the perk is retiring appeal court judge Sir Derek Cons. The second most senior judge in Hongkong, Sir Derek, 64, said he was looking forward to the cruise. ''After 38 years [of service in Hongkong], I deserve it,'' he said. Sir Derek said he would be joined by his wife and set off on the trip on March 4. His tour would stop in Southampton on April 7. Asked whether the cruise perk should be retained, he indicated that this was a matter for the Government. It is understood that other officers are still hoping the Government will drop plans to abolish the sea passage. Some are keeping their fingers crossed and have made a booking for the Canberra every year between now and 1997. The director of Architectural Services, Mr Paul Corser, will be eligible for the perk when he retires next year. Although the fate of the perk is very much up in the air, Mr Corser said last night that at the moment he would like to go. ''It would be next year,'' he said. ''I intend to go, but it's not certain yet. Maybe I'll change my mind.'' According to P & O Travel, the agent for the Canberra in Hongkong, the cruise will take a route passing Cape Town and making 10 stops, including Bangkok, Singapore, the Seychelles and Dakar before reaching Southampton. An alternative route for the cruise takes it along the Suez Canal. While there are more than 20 listed fares, civil servants usually travel in one of four classes, with the fare ranging from $38,484 to $64,128 per person. A spokesman for the travel agency said the fare determined the room size and facilities, but all passengers enjoyed the same services and recreational facilities on the liner. The cruise perk has recently attracted much publicity because the Government announced a plan to unilaterally scrap the fringe benefit for expatriate civil servants last year. Civil service unions and legislators challenged the decision, saying the Government was breaking its employment contracts, but the administration maintained that the offer was out-dated. The Government recently proposed two alternatives to the luxury cruise, one retaining the sea passage but freezing the cost at current price, and the other making an unspecific ex-gratia payment.