NHR and British taxation  

@vegasguideownmail-net

I'll have a stab at a couple of those.

We are completing our Residency before the Brexit deadline and as such will also be included in the NHR Tax Regime

Forgive me - I'm being pedantic here, just in case you may have overlooked something. You're not included in NHR by dint of getting residency prior to Dec 31st. Any new resident, from wherever, is eligible to apply for the status (but do note, it does need to be applied for as a separate process from registering residency - it isnt automatically conferred).

1. I have read a statement that Portuguese Taxation starts at 20%, but is there a personal allowance in the same way that there is in the UK before this kicks in?

No, that's not right about PT taxation starting at 20%. For benficiaries of NHR, qualifying income earned in Portugal is taxed at a 20% flat rate (but you have to be in the list of qualifying occupations in order to benefit from that).

The normal tax scales progress from 14.5% to 48% and for earned income and pensions there is a personal allowance equivalent, though nowhere near as high as the UK's, standing currently at 4,104€. There are some other allowances besides, though.

2. We have investments in ISA's in the UK, what is the tax treatment around those, are we able to maintain them or will there be some kind of penalty for entering the NHR regime?

Under NHR, you'd declare but not pay PT tax on the interest from those in Portugal. Without NHR, they'd be declared and taxed along with other income although you'd get a choice as to whether to have them taxed at a flat 28% or have them included with any other income and taxed at scale rates.