The regulations require the supply to a house to be split into circuits and there is a list as to why this is required which includes loss of lights should there be a fault.
We do break the rules, fitting aerial amplifiers in the loft and shaver socket in bathroom are often supplied from the lighting supply though strictly they should not.
So in theroy no you should not run the TV and/or sky box from lighting supply but in practice you will likely get away with it.
Problem is your unlikely to find a 5A extension cable with two 5A sockets. These have dried up in the UK. Likely can still buy them in Hong Kong I know area to look in and street name but in UK the only option would be 5A to 13A adaptor lead and this is where the trouble starts.
In the loft to supply a TV aerial amplifier no one is likely to try plugging in anything else, but for the TV the risk is too great so even if it will not blow the Fuse/MCB not really a good idea.
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