Sara,
I agree with the others. Don't bother with the hair dryer as most hotels provide dryers these days, either in the rooms or on request from the front desk. In addition to that, many of the high wattage Voltage Convertors use a solid-state conversion principle which produces a very rough sine wave and some appliances won't work well with that. The best Voltage Convertors use transformers, but those are inherently heavy so not as practical for travel with high wattage versions.
Regarding your flat iron / hair straightener, some manufacturers specifically prohibit the use of their products with Voltage Convertors. As I recall Chi is one of those manufacturers. It would be better to either buy a similar device in the U.K. as emma suggested or to just dispense with that for the trip.
Finally as Marco suggested, it would be prudent to use the correct U.K. Plug Adaptors. As I recall, this model meets all the requirements (it's fused and has the shutters over the hot terminals).......
https://www.amazon.com/Swiss-Gear-Grounded-Adaptor-Singapore/dp/B004MKMH5K