Thanks everyone for your helpful advice and guidance! You’ll be pleased to know that I cracked the foibles of AWS, and the Smart Home skill is now working! 
The issue was related to the Preferred Marketplace and Default delivery address for my account. The troubleshooting article above mentions this and interestingly I’ve not read this elsewhere or on the official documentation.
Most people probably won’t have this issue, if you’re using your everyday Amazon account. However, I created a new developer account for this purpose and I don’t use it for online shopping and therefore it didn’t have any regional information.
The fix was to login into https://www.amazon.co.uk and configure the home delivery address and then separately select the Default button even though I only had one address. Oddly it doesn’t automatically default. The locale region also requires setting under the payment page by setting your relevant country, United Kingdom in my instance.
For those in the UK, here is my final configuration that worked for me.
Lambda region = EU (Ireland)
Lamba instance = Ireland (eu-west-1)
Primary locale = English (UK)
Client ID: https://layla.amazon.com/
Alexa skill language: English (GB)
Alexa store language: English (UK/IE)
Home Assistant locale: en-GB
For an English speaker, in England, this configuration is completely bonkers! I gather if you’re using English (US) you won’t have these problems.