I strongly suspect so.

According to Robin Sterling's "The Weather in Britain", the lowest RH on record in the UK is 4%, at both Ringway and Great Dun Fell on 29 March 1965. Here's the synoptic chart:

http://i.imgur.com/UVS59Di.gif

However, I think that these are suspect as well, and the 8% alleged at Altnaharra on 18 February 2003 has since been discredited.

In "Weather" Vol 56, June 2001, Stephen Burt lists the lowest relative humidities in the UK since 1880, and has the very lowest as 9.5% at Parkstone, Dorset, on 24 May 1901 (citation: Hawke, 1944).

http://onlinelibrary.wiley.com/doi/1....tb06569.x/pdf

Synoptics:

http://imgur.com/55GoPKq

More recently there were relative humidities widely below 20% on 18 June 2000.

http://imgur.com/wg2mli3

The lowest was 11.5% at Brize Norton with a dry bulb temperature of 29.5 deg C and a dew point of -3.5 deg C at 16:00 UTC.

This is fourth lowest relative humidity in the UK record since 1800, equal with Parkstone again. In fact Parkstone appears three times in the top four.

Stephen.