I mostly do Perl stuff and get £400-500 day rate easily even though I'm far from a Perl expert (and, indeed, I loathe and despise the language nowadays.)
There's a few (senior) Go contracts starting to appear in London (at £500+/day) that I'm hoping signify the start of a trend.
Depends who you work for. A solid LinkedIn plus the subscription option, with stints in the big 5 and some solution architect experience gets you some decent money.
Similarly, if you can pick up the (relatively) obscure FinTech stuff (kdb+, Vertica, M/Mumps/Caché, etc.), that's a definite ticket to decent money and job security.
