Income Taxes - Schedule of Reconciliation between Effective Tax Rates and Statutory Rates (Detail)	12 Months Ended
	Apr. 30, 2022	Apr. 30, 2021	Apr. 30, 2020
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Income tax benefit at U.S. federal statutory rate	21.00%	21.00%	21.00%
Foreign rate differential	(1.50%)	(1.60%)	(1.60%)
Nondeductible expenses - UK R&D credit	(10.60%)	(8.30%)	(12.10%)
Other	(0.80%)	0.40%	(0.50%)
Valuation allowance	(8.10%)	(11.50%)	(6.60%)
Effective income tax rate			0.20%