does the compass lock in place?

Unfortunately neither compass in this kit lock in place.

what size lead does the compass use?

2mm diameter lead... 2mm in diameter - 16mm in length. The compass needs at least a millimeter of lead in the retainer part to be able to hold onto the lead, and that means that about 5mm of the lead is wasted.

Is this a suitable kit to substitute for an oxford geometry set?

Yes, it's very similar: Compact, comprehensive set includes metal compass with pencil holder, metal compass with lead, pencil sharpener, eraser, pencil, ruler, protractor, a set of squares, and one pack of leads. Packed in a reusable plastic storage box.Item PropertiesCategory: Drawing InstrumentsCategory: Ruling & Measuring ToolsBrand: Maped®Type: Drawing and Geometry Set