Mix in a bit of cockney every so often. Look up British/Irish/Welsh slang to toss in once in a while. Make them more reserved if they're rich but have them cuss like a sailor if they're poor. Watch Peaky Blinders, Downton Abbey, and Sherlock/Dr.Who for reference.
Whatever time period you use should be researched heavily. Also, make sure to add plenty of Margaret Thatcher hate if it set within the last 40 years. If it's modern day, watch some Sovietwomble videos on YouTube to see how they talk and interact.