Not worth lifting them tbh, doubt you'll find anything other than the standard cable bunches. I work for them and its safe to say they are pretty boring
I work for them too, here's a quick guide.
Concrete lid with GPO/BT/Telecom labelling is a footway joint box, inside it will be somewhere between 30cm and 2m deep with 4 inch ducts running in and out and a whole load of cable. Nothing terribly exciting and can be dangerous as they have a habit of filling with gas which can be ignited by a particularly enthusiastic lid lifter causing the metal parts to spark.
A rectangular iron lid in two triangular parts will probably be a carriageway joint box. Same as the above but very heavy iron lid for installation in carriageways.
A square iron lid with a cross shape is likely to be a manhole. Inside you can expect a ladder leading down into a chamber around 3-4 metres wide and long. Tall enough to stand in. This is essentially just a large joint box and could be a few metres underground. usually have a foot or two of water in the bottom and cable son the walls. Very very occasionally they will also have access to building basements and other underground structures (use your imagination).
