As most of the portfolio bond literature is written from a UK citizenship perspective, the terms 'onshore' and 'offshore' can be potentially misleading.
Technically, the product for your circumstances would be an offshore portfolio bond, but the broker (i.e. an authorised distributor) could be based anywhere.
Most providers of portfolio bonds only sell via intermediaries. This doesn't make any difference to the purchaser, as the embedded costs are the same.
Which brings me to the first key point: not all portfolio bonds are created equal. (i) The cost structures are designed to confuse and conceal a quick evaluation, and of course small % differences can have a considerable impact over time, (ii) there are other, more favourable, terms that are not advertised
Second key point: offshore brokerages are definitely not created equal. Caveat emptor....
With ETFs, you have a wider choice of purchasing channels. If you're a self-driven investor, you could use an offshore internet stockbroker, or possibly your bank if it offers a discount facility.
There are a large number of ETFs listed on the London Stock Exchange, which would match your GBP income.
Although an income fund may be ostensibly tax-free both in the jurisdiction where it's listed/sold and from your own tax situation, the underlying securities may suffer withholding tax on interest and dividend income (30% on US bonds, for example).
Possibly I may be able to help with more specifics ...send me a PM if you get stuck
melo