24Mb leased line is totally do-able.
Pricing I have seen puts a 10Mb line on a 10Mb bearer at around £7000 per year.
I haven't seen pricing for 20Mb, but to get 10Mb on a 100Mb bearer is around £9000 per year, this could be scaled up to 20Mb easly for little extra cost. I would imagine it would be less than a grand per month.
If you are a business and require the connection for all of your income and cannot afford any downtime, its often a price that is worth paying.
Also, a 1Gb leased line on a 1Gb bearer is around £160,000 per year, £13,333 per month, if Tesco are paying that much they are getting fleeced
All these prices are for BT Net, been to a few places which have this service...