With only 20k makes more sense to go DeGiro than IB anyway in terms of fees.
In terms of where to buy ETFs:
Buying in London will have 0.1% forex fee. In addition to this London has a 0.5% tax for buying and selling. London has no withholding tax.
If you buy SIX listed in CHF there will be no forex fee. There is no additional withholding tax in IE based ETFs listed on SIX. The Dividend will not be in CHF so expect forex fees there.
But my recommendation for this particular ETF would be to buy it in Amsterdam, because its on the DeGiro Free ETF list, but only if you buy in Amsterdam. The forex of 0.1% will apply, but this is a lot lower than paying the fee.
For your ETF its:
- Euronext Amsterdam
- SIX
- LSE