A few things to address.
Captains are earning around 138 Euro per hour, and most are working around 75 hours per month. Don't forget, on this much turn over, you would also have to be VAT registered. Try getting that out of BRK. They refuse to pay VAT monthly as they are supposed to. Payment is made quaterly, and that is only if the "Nice Mr Dooney" wants to send the cheque. Ask any VAT registered BRK pilot if they have payment for November yet!!
Going back to the "I even hear contractors are on SBY (= duty) for no pay". This is happening all the time. Some First Officers are working as little as 2 days out of a block of 5, ie, 2 days flying, 2 days standby, and 1 day allocated leave, then scheduled 4 days off. This effectively means 2 days in a 9 day cycle, and 6 days per month. You could be earning good pennies, (pennies being the word to focus on!)
With regards to working for any other aviation related organisation, or any other employment, you are correct in saying that this is an un-enforcable clause. The problem may come from the OPS manual at Ryanair, which states that you can not do any other work, and as per the BRK contract, you must follow the FR ops manual to the letter. So, here lies a small problem.
In the eyes of the revenue you would be classed as fully employed by Brookfield and liable to all the PAYE taxes and NI deductions, which if is enforced, then would be taken from your pay as per the clause in the contract.
IR35 is a ruling on how you run and pay yourself within a Ltd company. Check HMRC website for information.