Worth investing into an ETF on IG for cashback offer?

I have 20k on HSBC FTSE all world OEIC (I’d prefer to just stick to one fund, this is my preferred) however IG have a £300 cashback offer which is rather good, but don’t offer this fund. Is there downside to getting an all world ETF just to get the cashback? I was planning to put in £20k in IG, looking at VWRP, FWRG or SPDR ACWI. Thoughts on this? Can’t make up my mind. In the next tax year, I’d go back to investing in the HSBC.