BECTU is a trade union. The prices on their website are their recommended minimum rates. These are really good as a guide on what you can expect to pay for people who know what they're doing. You'll find cheaper but you just need one cock-up by a camera or sound assistant and your whole day's work is ruined.
When making a pitch for a job, always over-estimate. Be pessimistic and allow a bit for the things which will go wrong and for the things you've forgotten.

Remember, once you've got their interest you can always drop your price but you can't raise it without causing bad feelings.

Also never, never, ever believe someone who promisses lots of work for you later, providing you do a deal on early work. It never happens.