Your maximum load is very little indeed. You can safely stick 9 items into your 10way, then use the 10th outlet for the 2nd 10way to plug into like a daisy chain. Both need not plug in the wall. Its good practice to stick your higher power items in as close to supply as possible. That means your amp or tv will go in socket number1 of extension number1. Probably the tv unless you have you amp cranked up pritty high. I would imagine the tv is the noiseyest of all the devices so you wouldnt want it in extension number2 even if it were one of the lower powered devices. its noise would pass past the power outlets of all other devices. Computers are particularly noisey, and actually dump digital power/noise to earth/ground. a few pc's will knock out earth leakage breakers, there no joke. iirc you allow 5mA per pc, so 6 maximum per rcd which ofton means per house. Ofcource, diversity comes into play. Like your 13 devices, there not all in use at the same time.
Hopefully that clears things up. oh.. the pun.
The 2nd extension wants a fuse lower than 13amp. its just a nice thing to do. it helps when things go wrong. If a fault occurs on the second extension, like a short, then as currant rockets upwards into the high figures, you want the 2nd(faulty)extensions fuse to blow. With a 13amp fuse in each the first extensions fuse would blow though. it would see both the same fault currant, plus the items pluged in to its self that dont run through the faulty extensions fuse. It would be the wrong fuse that blew. safe... but bothersome to find without fuse discrimination. I would stick a 3amp in it with such a low load. A 3amp also covers the fact i dunno how long this first lead is, how fat it is, its fuse size, protection, nothing. I doubt you have a ridiculously long set of extensions though. They sound professional enough. If they reach right round a shop though then i would want to do the math before using them at all. Its non of the math in this thread, and im not going to attept to explain it unless asked. most sparks just pick up a good practice guide rather than fully work stuff out.
uk mains is 230v by the way. +10% / -6% tollerence. All maths is based on 230v now, and 230v sumations and ratings allow for the tollerences. Some publications are yet to fall in line, but its on the cards.