Is investing in LON S&P 500 VUSD better than S&P 500 VOO in terms of tax?

Yup, that is correct, the tax is known as the withholding tax on dividends and only applies to dividends. If your broker allows LSE access, then you should select the Irish Domiciled ETFs. However, do also note that US ETFs tend to have higher liquidity due to the higher volume traded so that might be a point of consideration for some. (Although for S&P 500 ETFs, the liquidity of Irish Domiciled ETFs are quite sufficient anyways).

The tax is only on dividends. Any capital gains, as a foreign investor, won’t be taxed.