Lynx and Degiro can be cheap too but both of them are Dutch so the deposit protection per client is €20,000 only.
I am also looking at Lynx, seems a very interesting option.
About the protection, i found this on their website (translated via google translate from Dutch to English).
"Your money balance is well protected via LYNX. Since you hold your account through LYNX at IB UK, you are therefore eligible for the protection of the U.K. Financial Services Compensations Scheme (FSCS). This means that your account is protected up to GBP 50,000. For more information about the FSCS, go to www.fscs.org.uk. More information about FSA can be found at www.fsa.gov.uk. This system covers a potential shortcoming or insolvency of the broker and as a matter of course does not cover loss on your securities transactions.
IB UK is part of IB USA. Clients of LYNX therefore also benefit from the protection of the American Securities Investor Protection Corporation (SIPC) and of the British insurer Lloyds through IB USA. The SIPC is the self-regulatory institute for Brokers / Dealer Members in the United States. Almost all Brokers / Dealers in the United States are members of the SIPC. The SIPC will compensate investors who have an account with an affiliated Broker / Dealer in the event of insolvency, bankruptcy or unauthorized actions of that Broker / Dealer. Indemnifying means that the market value of your shares, options, warrants, debt and money balance is protected by insurance up to $ 500,000, of which $ 250,000 is cash (or the equivalent amount in EUR). If the SIPC coverage is not sufficient, Lloyds will insure up to an additional $ 30 million per Client. (of this $ 900,000 cash). This insurance is limited to $ 150 million in total."