The lucozade drink will have the same effect a a gel, but you need to drink the whole bottle to get the same amount of energy as the gel packet. This means gels are much easier to carry etc when running (this is their main advantage).
Coincidentally, I just had a similar (but less severe) reaction to High5 isogel a week or so ago. I bought the marathon pack from High5 and was testing the various flavours on a long run (with no real problems). The only one that my stomach struggled with was the isogel.
For a half marathon, I wouldn't normally bother with energy products. However, if they give out an energy drink at a water stop then I'll quite happily take it. You don't have to worry about drinking it all, but whatever you manage to drink will be a small energy boost and a bit of refreshment.
Have you seen anyone about your knee? Its getting close to the event, but a physio/sports therapist may be able to relieve the pain enough to let you participate. Most people will pick up aches and strains during training, and its sometimes useful to get someone else to assess how serious they are.
As cougie said, very few (if any) people would ask for the charity contribution back if you don't take part. No one would want you to get seriously injured during the event, so people understand that you have to pull out of events.
An alternative would be to use the Shoefitr online application to see how it compares to your existing shoe.
If you go to the runningwarehouse.com web site and find the guide 7, then click on the "show me how it fits" option. This will show you how it compares to your existing shoe (NB as its a US site, the sizing is american rather than UK).
You can't buy from here as they are US only, but you can then translate this into the correct UK size for you. I found it useful when I got a pair of very cheap Saucony Fastwitch (coincidentally from Wiggle!) as the shoefitr advised to go up half a size from my current shoe, which was also a Saucony.
As 198bpm is the highest, I'd use that. You could round that up to 200 bpm as I suspect you could have (theoretically) gone a bit higher unless it was during an all-out sprint at the end, or whilst climbing a steep hill on the course.
As you've said these weren't abnormal spikes in the trace, they all sound genuine readings.