Looking into the Saquerium I can see that His Grace, The Lord Bishop of London is already present, it looks like he arrived with the Eccumenicals
Just to be clear, the Saquerium is the east end of the Chancel between the Sanctury (which contains the High Altar) in the East, and the Quire in the West
Westminster abbey is unusual in that its Quire begins BEFORE its Transepts...so the Saquerium begins directly after the Crossing, and bizzarly, it also has a Quire Screen that extends half way down the Nave...thats unusual in a Cathedral, that almost always either has a completely enclosed Quire containing everything up to the Rearadross and then a large Retroquire before the Apse...OR they have an open plan Quire, and next to no Retroquire at all.
Almost all Retroquires are converted into Chapels...in Saint Pauls the Retroquire is very small, and its been converted into The American Memorial Chapel infact...these chapels end in the Apse, which is the Eastern curved wall of the Cathedral
You begin with steps at the West End, followed by an optional covered porch, and the Great West Doors, inside you find the Vestuble, and in some Cathedrals, you also find a Narthex, one of the biggest is inside Liverpool Cathedral, its SO large its like a proto-nave its VAST. Beyond that you have the Nave, and going upwards in the Nave you usually have a second tier which is known as a Triforium, where there are big Galleries, probably a library aswell, above that you have the Clerestory which is a tiny high up walkway around the nave, below the Nave you may have a Crypt, or a Charnel House, or an Undercroft...and Undercroft will extend beyond the confines of the church above and probably have external exits outside, Charnel Houses are like Mausoleums, except inside a Church so they will contain caskets, and a Crypt is probably mainly filled with monuments rather then bodies.
at the west end of the Nave you will find the Crossing, this will extend north and south forming the Transepts, and entries to the circular Chapterhouse if the Cathedral is old enough, and maybe the Cloisters if the Cathedral used to be a monastic foundation. North and south Asiles probably run along the Nave aswell, and there could be side chapels.
Any Dome or central tower will be placed right on top of the crossing, and any constructed Altar for the Nave is likely to be placed on a movable Dais at this point.
Beyond the Crossing, further East you find the Quire. Sometimes this has a screen with a gate which you have to pass through, sometimes there is no screen. Either way, there is usually an Ambulatory, which consists of two aisles, one north, one South which go around the outside of the Quire and meet at the Apse. Inside the Quire you have the Stalls, at the East End of the Stalls on the South side, you will find the Cathedra Throne (in a Roman Church the Throne is often right at the East End, touching the Apse. Beyond the throne will be the area between the stalls and the Communion Rail...that is the Saquerium. After the Communion rail you will find The Sanctuary, and together the Quire, Saquerium and Sanctuary compose The Chancel of the Cathedral.
East of that will be the High Altar, it will either be sitting under a huge canopy type structure, known as a Baldekino, OR it will stand infront of a decorative wall covering, which is known as a Rearadross.
Behind the Rearadross will be the RetroQuire...the space behind the High Altar, this will probably be converted into a Chapel which will touch the Apse and form part of the ambulatory
I hasten to add that there will be an many rooms which come off the nave and the ambulatory, called Vestory, that are preparation rooms for priests, and one very important room which comes off the Ambulatory will be known as The Sacristy, which is the preparation area for Communion stuffs, Cyborium, Chalice, Plate, Wafers, Wine, monsterones possibly some Reserve Sacrement
The place will likely also have a Treasury, or at the very least a Strong Room, where it stores valuables like processional crosses, candels, and it will have some place where the Vestments are stored, all the Chasubles and Copes and the likes. It may have Tabernakles which are like a Safe built into the wall of chapels where the Host can be stored, it may also have a pyx...these are strange looking objects that hang from the ceiling suspended above chapel altars, the Host is kept inside and can be gained by lowering the pyx, this is most often where any reserve sacrement is kept