[How do I buy bonds on my ISA or Share Dealing account?]

Hi all. I have a chunk of cash sitting around and don't really want to allocate it to equities or anything too risky so I was hoping to pick up some regular coupons from something like non-junk bond.

I've tried to find bonds on my ISA or Share Dealing account but they don't really seem to exist. I can't find bonds nor ETFs of bonds... 

Also can someone help me to understand - like I don't want to bet on the value of bonds or rates going up. I only care about dumping cash in bonds and receiving regular coupons. I understand that the prices of bonds go up and down and therefore you may make a gain/loss on your principal if you sell before maturity, but I kinda want a low-risk bond and want to hold it to maturity. Or at least something that has that kind of risk-reward profile.

[Every time I try to transfer my ISA from HL I get an error]

I have a Stocks and Shares ISA with HL, in which I've sold all my holdings and only have cash. All contributions made in that ISA have been in previous years.

I have tried twice this morning to fill in the ISA Transfer form on IG, with all the correct details... of course I don't have any holdings to fill in because I have sold them all.

When I click the green button saying something like "Details are correct", I get an error saying "Something went wrong. Try again"... and I have no idea how to progress from there.