Thanks for the recommendations. The Viessmann 111-W seems like an interesting option. Only thing, like you mentioned is the size. I would need to double-check whether this would fit in the space we are intending to place the boiler. The boiler also seems to fall under the combi section, but with it having a built-in storage cylinder, would the thermostat need to be a 2 channel option (for heating and water) or will it work on a single channel one? Asking as I have already purchased the single channel version of the Drayton Wiser smart thermostat which is supposed to be for combi boilers. I suppose I could consider the Viessmann 100-W instead, but the built storage cylinder could be helpful.
I'd rather stay away from Vaillant. I know they are supposed to be fairly highly regarded and I have been recommended the EcoTec Plus model by a heating engineer friend. Based off the experience my parents had though, I have discarded them as an option.
I'll have a look at the Intergas models but it seems that there aren't many distributor options to purchase them.