Is the tax-free allowance applicable per item or per total cost of all items combined in a baggage

The official UK document says:

You can bring in other goods worth up to £390 (or up to £270 if you arrive by private plane or boat).

If a single item’s worth more than your allowance you pay any duty or tax on its full value, not just the value above the allowance.

The question:

Is the tax-free allowance applicable per an individual item or per total cost of all items combined in a baggage?

If you need a more detailed example, here it is.

I am relocating to the UK and going to take my desktop computer with me. I want to disassemble it and put small parts into my backpack and big parts into a suitcase that will be put into aeroplane's luggage. All these parts will be with me upon my arrival to the UK.

As for the computer. I bought all its components separately in my country and put them all together and used the computer for around a year. Almost all the parts in the desktop were cheaper than the tax-free allowance except the Video Card.

What will I be charged for? For the sum of all the components prices or only for the video card?