Q:

What should I put the remainder of my cash in? 

I currently have 7 shares of CSPX.L and 150 shares of IWDG.L. I believe these to be sound investments but this is a belief backed up by little financial understanding.  I think these ETFs are good because they diversify my portfolio geographically and by sector but, as I understand, are vested entirely in equities. 
I was thinking of buying £1000 worth of an ETF in government bonds (IGLT specifically).  I did a bit of research and I've heard of them described as an "insurance policy" against equities leading me to believe that generally when stocks go up, bonds go down and vice versa.  Is this the most economical use of my cash?  Can I expect the two to be somewhat inversely proportional to each other? 
EDIT
Based on a comment I'm adding more information.  I would say I have a slightly above average tolerance for risk.  On a scale of 1-10 (ten being riskiest) I would be a 6.  
In terms of investment goals I don't have any specific target.  I just have some money that I want to put to better use other than an ISA.  I may withdraw it in 5 years, 10 years or 40 years.  

A:

Your question should probably be closed because it is broad and opinion based.   However, I am encouraged by the way you asked it because you are not failing for some get rich quick scheme (like FOREX) which will end up leaving you with less money then when you started.   
Only with hindsight can one answer what the most efficient use of your cash would have been.   
You are teetering on going down the path of "asset allocation" this was popularized by Jack Bogle of Vanguard.   Something like 60-90% of your money in equities, with 10-25% of that being international investments.   The rest would go into bond funds.   Some really like this approach, others disagree.   It is hard to imagine who is correct and who isn't.   I should point out the equities that Bogle promoted were in low cost index funds, not those that are actively managed.   
For me, and I am in my 50's, I tend to use a pretty aggressive asset allocation model with about 80% in stocks.   I use a mix of both actively managed and index funds.   I almost exclusively use bond funds, not buy individual bonds.   Is this the most efficient use of my investments?    It has not been, but I have done pretty well, and I did not own an item that would have enabled me to predict the future.  
So to answer your question in the general sense, I think you are on the right track.   In the specific sense it is impossible to answer your question and only the charlatans will tell you that they can answer specifically.   Don't trust those people.   
Keep learning about investments and adjust your portfolio from the information received.