GREAT BRITAIN THE LOSS OF LUTON THE BOOMING AUTOMOBILE-MAKING TOWN OF LUTON, TORY PARTY CHAIRMAN JOHN HARE DECLARED RECENTLY, IS " A MICROCOSM OF THE BRITAIN WE ARE BUILDING . " IF SO, IT MAY BE THE SOCIALISTS WHO WILL TAKE OVER THE CONSTRUCTION JOB . AT A BY-ELECTION LAST WEEK IN LUTON, 30 MILES NORTHWEST OF LONDON, VOTERS ELECTED A LABOR M.P . FOR THE FIRST TIME IN 13 YEARS, TURNING THE TORIES' 1959 MAJORITY OF 5,000 VOTES INTO A THUMPING 3,749-VOTE MARGIN FOR LABOR . THE SWITCH, PRONOUNCED LABOR PARTY LEADER HAROLD WILSON TRIUMPHANTLY, WAS CLEAR PROOF THAT " THE CONSERVATIVE GOVERNMENT HAS TOTALLY LOST THE CONFIDENCE OF THE COUNTRY . " NEXT DAY THE TORIES HAD ONE TO TALK ABOUT, WHEN BALLOTS WERE AT LAST COUNTED AFTER ANOTHER BY-ELECTION IN THE SPRAWLING SCOTTISH CONSTITUENCY OF KINROSS AND WEST PERTHSHIRE . THERE, IN ONE OF BRITAIN'S SAFEST TORY SEATS, TORY PRIME MINISTER LORD HOME NOW PLAIN SIR ALEC DOUGLAS-HOME WON A SEAT IN THE HOUSE OF COMMONS . HIS 9,328-VOTE MARGIN EXCEEDED HIS PARTY'S MOST BUOYANT EXPECTATIONS . WHAT'S MORE, IN THE COURSE OF 72 SPEECHES AND A HECTIC ELEVEN-DAY CAMPAIGN, THE FORMER PEER PROVED THAT HE IS A VIGOROUS, TOUGHMINDED POLITICIAN WHO SEEMS WELLEQUIPPED TO HOLD HIS OWN IN PARLIAMENTARY FREE-FOR-ALL . HE WILL HAVE TO . IN THE KIND OF MARGINAL SEAT THAT MATTERS MOST, LIKE LUTON, TORIES ARE FARING BADLY . WHAT TROUBLES THE TORIES IS THAT LUTON IS A SIGN OF TORY AFFLUENCE, WITH INDUSTRIAL PAYROLLS THAT HAVE BOOSTED WAGE LEVELS 20 PER CENT ABOVE THE NATIONAL AVERAGE . LUTON HAS A BIGGER-THAN-AVERAGE SHARE OF THE FASTGROWING MIDDLE CLASS THAT HAS KEPT THE CONSERVATIVES IN OFFICE FOR TWELVE YEARS . LABOR CANDIDATE WILL HOWIE, 39, A NEAT, BESPECTACLED CIVIL ENGINEER, WON OUT OVER TORY SIR JOHN FLETCHER-COOKE, 52, A TWEEDY, MUSTACHED FORMER COLONIAL ADMINISTRATOR, BY PROMISING LUTON THE NEW SCHOOLS, HOUSING AND INDUSTRIAL EXPANSION THAT LABOR IS PRAGMATICALLY BUILDING ITS ELECTION HOPES AROUND . BEFORE RETURNING TO LONDON FOR PARLIAMENT'S REOPENING THIS WEEK, DOUGLASHOME, THE NEW M.P . FOR KINROSS, REMAINED PROFESSIONALLY OPTIMISTIC : " LUTON WAS THE LAST PAGE OF THE OLD CHAPTER . KINROSS IS THE FIRST PAGE OF THE NEW . /