Post Brexit (01/Jan/2021) postage issues between EU and UK

If you send your own personal effects into a country, there are forms you can fill in to negate all tax. But you will have to prove residence in the country you are sending them to.

In the case you have outlined, the correct procedure is gift, with nominal values per item, £2-10 kind of thing. The recipiant may have to pay import duties and things, but if the total value is below I think 22 Euro they wont have to pay any fees.

So:

Gift,
Bike Parts, 6 pieces, £2 (per item) total value = £12

You shouldn't really need a HS code for that, but if you want To be flashy you can uuse 87149990

I hope this helps.
Brexit is an absolute disaster (as we all knew it would be in 2016)