Ignoring the initial healthcare needs as those have already been covered.
You're introducing a virus to the system. A virus is most likely to kill or harm the vulnerable, the already sick, the elderly, and the very young.
Why do I emphasise this? because these are not productive people, they are the people already dependent on society. By killing off all these people you're increasing the economic output of the average person. You're freeing up long term resources from hospitals and care homes. You're streamlining the economy, reducing the load from the cost of pensions, benefits etc.
In short, by using a virus in this way, you're likely to strengthen rather than weaken the economy.
Looking at the ONS stats and assuming that you're going to kill off the unproductive, unhealthy, and elderly first. Population is around 65million, working population around 30million. You might have to kill off 50% of the population before you start to effectively weaken the productive capability of the country.
Until that point, all you're doing is freeing up labour from tasks related to supporting society's dependents. You're also freeing up funds from paying for their needs, their benefits and pensions, and their healthcare.
The rates of consumption are another matter, Keynes says we're driven by consumption not production capability and as such the idle 50% are pure consumers. They drive the economy by their needs, without them, who is consuming what the producers generate? On this model the economy starts to fail rapidly in proportion to how many of these consumers are removed from the system.
This however is only a local consideration, much of what is being produced is being consumed overseas not locally. The knock on effect of stripping a large percentage of consumers out of the system can only be measured on a global scale. They are consuming products from across the world, the effect would be a drop in the trade deficit and a lot of complex calculations relating to the values of currencies.
We haven't yet managed to put a big dent in a single economy but we might have triggered a global recession. We're back to having to kill off the producers, the toughest and the healthiest and the last to die of a viral outbreak. At the upper end, you could be looking at killing off well over 50% of the population.
There's actually a way out of this problem. There's a range of viruses that kill as much by immune overreaction as by the effect of the virus itself. Spanish Flu being a key example of this. Not just because of how many it killed, but because of who it killed.
An unusual feature of this pandemic was that it mostly killed young adults. In 1918–1919, 99% of pandemic influenza deaths in the US occurred in people under 65, and nearly half in young adults 20 to 40 years old. In 1920 the mortality rate among people under 65 had decreased six-fold to half the mortality rate of people over 65, but still 92% of deaths occurred in people under 65.
A virus like this could rapidly cripple the economy, but you're unlikely to be able to contain it enough to protect your own people.