From my experience as a Brit and also spending my early days within the UK player market is that the players do tend to wager smaller amounts, and getting a whale is harder.
On the up note UK people are becoming more adventurous and trying out online gaming, and do not have the restrictions on the US audience (such as making a payment online).
Things in my experience as a UK players and what I feel they are looking for
Well recognised and named casino such as Victor Chandler, William Hill (this is because it is something they are familiar with.
Emphasis on casinos which cover common UK casino games such as BJ, Roulette, Lottery and Slots…
Easy payouts, and no complex red tape
Things in my experience as a UK player and things they tend to avoid
High Bonuses which seem to high to be realistic (i.e. they tend to not go for bonuses higher than 100% match play.
Long winded terms and conditions within casinos.
Casinos which say they cater for the UK audience and then use US lingo within the site.
Casino’s which tend to put a large focus on the US audience and US holiday dates (UK audience like xmas, bank holidays, easter and new year)
Casino’s which tend to focus on US games
Also Brits love to gamble on the races!!! and events such as the Grand National are big days for everyone in England!