The UK has two tax-efficient schemes for investing in startups: SEIS and EIS. For some perspective, if an 'additional rate taxpayer' invested £10,000 into your startup, they are only actually risking £2,750 due to tax relief (source: https://www.crowdcube.com/pg/seis-tax-relief-42). They get additional relief on capital gains tax.

Basically anyone with high income, making more than (a few) £100k per year.

I have a friend who has done a couple of SEIS investment deals that turned out ok-ish (only breakeven, but profitable considering the tax rebate mechanism).