You can ask MetaQuotes about any their price feed ... as I know - the brokers are having their own price feed sources.
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As to backtesting so we are having good summary about it (for MT5 only sorry) so you can read with all the particularities -

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Hi guys,
Are there any brokers that are using the metaquotes price feed?
I've been doing some backtesting using the metaquotes price feed from the History centre, and Metaquotes themselves do not provide a brokerage service. Any advice would be highly appreciated and apologies if this topic has been already covered before- I am new to this forum and have run a search but I could not find what I was looking for.