Hi,
You can see guidance here:
IFM03350 - Authorised investment funds (AIFs): taxation of investors within the charge to IT: distributions
CG57707 - Unit trusts: accumulation units
The first link provides an answer to the questions on the taxation of dividend and interest distributions. As you can see, these should be declared in the same way as any other dividend and gross interest payments. In this case, the payments will have been made from an overseas unit trust and should therefore be declared on the appropriate sections of the foreign income pages.
The units in the unit trust are treated as if they are shares in an ordinary company. The capital gains treatment of accumulation units is that when notional distributions, as described at CG57707, are received and are subject to Income Tax, the amount of those distributions will be allowable additional expenditure in a capital gain computation following a future disposal. As stated, this treatment applies to both UK resident and non UK resident trusts.
Thank you.