I agree that comparisons are difficult.
The tax rate on interest is at your marginal rate but there is no tax on interest on deposits held within a Cash ISA - to which you can contribute a maximum of £5,760 (for 2013/4) per year.
So you get 2.5% tax-free on smaller annual investments within an ISA but, for £100,000, you'd want to be looking at fixed rates of:
1 year: 2.15%
2 year: 2.45%
3 year: 2.51%
5 year: 3%
All are subject to 20% tax - or 40% if you're a higher rate tax-payer.
Mortgages are even more difficult given the range of providers, the rates themselves and the WIDELY varying arrangement fees. It's impossible to compare mortgages without taking the amount to be borrowed into consideration (do I go for a high fee and a low rate or high rate and low fee).
To give you an example of the products I've taken over the past 6 months:
- 2.3% Tax-Free ISA Deposit with Tesco Bank at 2.3% using last years ISA allowance. The deposit remains there earning 2.3% tax-free
- 3% Regular Savings ISA using this years ISA allowance to which I'm contributing £480 per month
- Ulster Bank Mortgage with no arrangement fees at a 2-year discounted rate of 2.9%, reverting to 4% SVR
The Ulster Bank mortgage was at 75% LTV and the rate on that for new applicants is now 3.3% - or 2.8% at 60% LTV.
As you can see, these figures definately wouldn't stack up for a new mutual society. I'm better saving in my 3% regular saver ISA than overpaying my mortgage as it pays 3% tax-free versus the 2.9% rate on my mortgage.
I believe this is all down to Funding for Lending - as the UK can print their own money and didn't give up their control over their currency in joining the euro, the banks can all lend dirt cheap money from the government and have little reliance on deposits. That's the reason a 5-year fixed rate deposit outside an ISA is as low as 3%.