IS the British Embassy in Beijing a postbox? Those of us brought up on BBC television, and old enough to remember Dr Who and his spaceship disguised as a police telephone-box, may attach more significance to this question than it perhaps deserves. But both Martin Barrow, who said it was a postbox, and Secretary for Security Alistair Asprey, who said it wasn't, are British gentlemen of a certain age. They are just young enough, however, to have fond memories of the time-and-space travelling doctor in the checked trousers and the novel conveyance. Neither can therefore be unaware that a spacious, comfortable but functional accommodation can be packed into an area no bigger than a shower-compartment. (If you don't believe it, ask any Hong Kong property agent . . ) When such men lock horns over whether an embassy is or is not a letter-box, we have to take the matter seriously. And when Howard Young, who was a student in England in Dr Who's heyday, says Mr Asprey has admitted the embassy is a postbox just seconds after he has firmly denied it, we begin to realise there is more to this exchange than meets the eye. The truth emerged, as Mr Asprey said he didn't say, but seemed to be saying (which is just as good, if you ask Mr Young) that this was no ordinary postbox. This was a super-postbox which could answer inquiries on Hong Kong visa requirements as well as hold letters until the postman arrived. And just to prove what a brilliant little device it really was, Mr Young wanted ''the British post-office embassy'' to provide performance pledges just like any civil service department. At this point Legco President John Swaine intervened, suggesting it might be out of order for Mr Young to demand a Hong Kong official like Mr Asprey issue performance pledges on behalf of the British Government. But the Secretary for Security obviously liked the idea of parking a British postbox in Beijing, bright red and marked with the symbol ''EIIR''. Gamely he offered to ask for a pledge anyway. Secretary for Planning Environment and Lands Tony Eason probably wished Dr Who's time-machine could fly him away. Zachary Wong first asked him to explain in detail what the Government intended to do about property prices, what level they should fall to and when this would happen. Then, when Mr Eason said it was all up to the newly formed property market taskforce to look into, Mr Wong said his answer was a ''misuse of words, a waste of paper'' and could be summarised as: ''No comment.'' And the worst of it was that Mr Eason knew he was right. Mr Young then made a statement on supply and demand. But it had a question mark at the end, suggesting he might be asking whether the Government would be concentrating on boosting supply or curbing demand. ''I think that is a question,'' said President Swaine generously. ''I admit it is a question,'' conceded Mr Eason. It was only when Lee Wing-tat, the man with the worst haircut in the house (not including those members with wigs), stood up and asked if the Government's planned curbs on property scalpers would last only a few months or were meant to be permanent, that legislators got a full answer. ''The tone of my replies will be interpreted by some members as negative and defensive,'' said Mr Eason. He was right.