i know bitmex has not with 1:100 but PL calculated on the moving average line ,not market
I don't use spread betting and CFDs very often for that very reason. I was just suggesting comparing the financing costs with IG because they are a proper brokerage and that should give a good feel for what the rate should be. If you're looking for a medium to long-term swing trade as the OP is then a bit of extra cost in the spread could easily be outweighed by financing costs. If you're looking for day trading then really I would recommend staying away from all spread betting and CFD's as the spread cost will kill your profit compared to paying commissions on an exchange.
At the end of last week, the Chicago Mercantile Exchange (CME) announced that they plan to introduce a Bitcoin futures contract at the end of this year or early next year. It is still dependant on regulatory approval but if they get it and you're a day trader this will definitely be the best way to go.