You have a £12k budget? Is that for everything inc future mods?
Adding forced induction to a hr costs about £12k+ on its own but you can get 500bhp.
On a de you could reach 450 for half that. Above that you need forged / stronger engine internals etc.
So my question would be what's your end game?
If I could start again, I'd get a pre 06 cheaper tax 300 so I could afford to supercharge. I thought the date was 23rd march 2006.
https://www.gov.uk/vehicle-tax-rate-tables