SELLING of Hong Kong stocks continued into London trading time last night with key Hang Seng Index components losing significant ground. The losses on the London Stock Exchange indicated the Hong Kong stock index will be at least 50 points down at the opening of trading today, based on the most conservative calculations. ''Stocks were off by more than two or three spreads, in some cases significantly more as institutions took profits,'' a dealer with James Capel said. It appears more than two weeks of record closes have been truly broken by yesterday's selling in Hong Kong trading, taking 169.72 points or 1.88 per cent off the Hang Seng Index to 8,861.41. According to London dealers last night, the index could slip beneath 8,800 in the first moments of trading, unless major bargain-hunting strikes in the first moments to restore upward momentum in stock movements. HSBC Holdings was down about $1.50 last night to around $86, from yesterday's $87.50 close in Hong Kong. Hongkong Telecom was 30 cents lower at around $15.70. These two stocks make up 26.6 per cent of the index and therefore have a significant impact on point gains and losses on the index. Last night's losses amounted to almost 40 points alone. Cathay Pacific Airways traded down around 20 cents to $12.20, Cheung Kong was off more than 70 cents to $32.85 and Wharf lost ground amounting to some 40 cents to $26.55. Hongkong Electric is expected to show a loss of 60 cents overnight to about $24.10 and Hongkong Land is off 20 cents to around $18.85. ''The losses appear to come in selling across the board. Finance has been hit quite hard and there is some concentrated selling of property counters,'' said the dealer. Hang Lung was down 35 cents to around $13.35 and New World Development saw similar losses at $23.85. Henderson Land was expected to show a loss of 50 cents to $29 and Swire was trading down more than 60 cents to around $47.70. Hutchison Whampoa was down about 35 cents to $28.05, Jardine Matheson about $1.50 weaker to $71 and Sun Hung Kai Properties off about 50 cents to $46.85. ''Only very strong and concerted buying at the opening will save the index on Wednesday. It will take a lot of firepower,'' said the dealer.