In the UK it is normal to quote salaries gross, pre-tax, but you're going to pay 40% on anything over £43k - and that's about right for a mid-level dev in London - and 45% on anything over £150k which is mid-size CTO territory. However, advertised rates are always pre-tax.
As to how to get a better idea of rates, genuinely, self-reporting is BS. Looking at advertised rates is the best way to get a sense of trends. http://www.itjobswatch.co.uk is used by everybody I know. Note that salaries in the North/Scotland are much lower than in London. A mid-level dev salary in London is potentially high-end startup CTO/Head of Engineering in Manchester or Leeds.
Not discussing/disclosing salaries is common in the UK, and when people do, they often low-ball it, IME.
I have always considered people's reports of earning $200k as being a little… optimistic.