The House of Lords -- where the affairs of a nation are most courteously debated

Slowly it dawns on the sightseers waiting in the Peers' Lobby that the police are making a pathway through the crowd. Then off in the distance the procession approaches, tiny figures dwarfed by the enormous ceilings. First comes the mace bearer, then the bewigged and robed figure of the Lord Chancellor, followed by the train bearers.

''Strangers, hats off,'' the police command, whipping off their own helmets and tucking them under their arms. So the Lord Chancellor, Speaker of the House of Lords, proceeds to the chamber and takes his place on the Woolsack, a most uncomfortable-looking enormous cushion, filled with the fleece of sheep from England, Wales, Scotland, Northern Ireland, and the Commonwealth countries. An ancient symbol to remind Britons what their prosperity rested upon, it affords the poor Speaker no support at all.

Though they act as a kind of second chamber, the Lords cannot veto ordinary legislation passed by the Commons - only delay it. Their importance lies in their careful debating of every bill and the amendments they send back to the Commons.

If everyone entitled to attend actually arrived on the same day, the chamber would have to do a giant stretching act to accommodate two archbishops, 24 bishops of the Church of England, and over a thousand peers (about 50 are women). About 350 have been given their titles for service to the nation. Fortunately only about 290 usually turn up to debate the affairs of the nation, with great formality and exquisite courtesy. It is not history reenacted - it's history continuing and well worth witnessing.

Of course anyone can line up to watch the debates in either house. But it can be a long wait, and only a small proportion is admitted. But there is another way - get in touch with Caroline Marshall, the wife of an MP.

With the House (and her husband) sometimes sitting all day and long into the night, Mrs. Marshall found there was something useful and pleasant she could do: take overseas visitors into the Houses of Parliament (no queues) or on other tours (the silver vaults, Big Ben, unusual houses, for instance). Best of all, perhaps, she will invite them to home-cooked lunch at her 18th-century house behind Westminster Abbey.

Practical information: If you want to witness a debate and do not have an introduction from an MP, arrive at St. Stephen's Porch by about 1:30 on a day when the house is sitting and hope to be one of the few admitted for the 2:30 debate (the Lords is easier to get into). Or, if you are an overseas visitor, your embassy may be able to give you a special ticket. Or Caroline Marshall can be reached through the Tourist Information Center, 64 St. James's Street (499- 9325) or through Mrs. Stevens, telephone Fittleworth 221. (Mrs. Marshall charges about $20 an hour.)