Credit Rating of a nation does affect the populace in indirect ways.
Credit rating is a direct reflection of a nations ability to repay its creditors. The lesser the rating is, the lesser the nation receives investments (Considering risk free interest rates)
If a nation's credit rating is downgraded, the investors/those planning to invest would reduce their stake, which impacts the investment levels. Which implies less money into real estate, stocks, assets and other areas, which indirectly means, less money flowing to the government in the form of taxes, which impacts their ability to spend. In case of stocks, their Price wouldn't appreciate to anticipated levels, which means the less net worth of the nation.
The less the money being spent by the government, the lesser resources you have on streets. Money just changes hands. But the major spender is the government. The less they spend, the lesser it flows in the streets. Expenditure is always a rising component in any society. With less money changing hands in the streets, something must take a hit. It's often the quality of the products and services offered. if the problem still continues, wages and employment levels take hit.
If the UK is further downgraded, the pound will depreciate a lot. Which means the cost of products/services will go up, with the wages remaining the same. What would happen to those on the streets? Cost of daily consumables goes up by ~10-15% from the same wage that they had been receiving. Which impacts the consumption of a nation. The UK is a trade deficit nation, so the outflow is always higher than inflow. This difference exacerbates and will continue to rise.
If you see, the rating downgrade is the first step confirming a nation growth is in downward trajectory. Indirectly implying to the companies/rich to start moving out & stop additional investments. If someone brings in 100 pounds and because of the rating downgrade, if the pound crashes by 5%, the investor has only 95 pounds now. Why would he bring in his money? What would he do? He just moves out his money to protect downfall. Corporates borrow additional money and move everything out. Once the currency is devalued, they bring back some of it and pay off their debts. They get to keep the difference.