You may want to ask the IRS - they have a desk at the US Embassy in Grosvenor Square.
As others have explained owing tax (or not) is not the only factor in determining if the IRS, FINcen or the Department of Justice may choose to levy penalties. No-one here has seen these returns (apart from you) so simply asserting that no tax is due may be factually incorrect as we don't know the facts.
The kond of issues a professional would ask would include questioning if there money in ISAs, was there other money that would be tax-free in the UK, were there foreign currency gains, were there gifts from a spouse, was there money in UK pension plans?