Investments in UK gilts (government bonds) only count if they were made before March 29, 2019.   After that, the investment is considered outside and in addition to the Tier 1 Investor Visa requirement to invest at least £2 million in shares or loans to UK companies.   That means you can invest in gilts, but the amount is not added into the £2 million or more portfolio needed to make a Tier 1 Investor Visa application.