There was always a grey area regarding the treatment of accumulated income and capital in relevant property (discretionary) trusts and it was left to the courts to sort it out. However, in 2014 the law was changed to "clarify" the situation - see for example
https://library.croneri.co.uk/cch_uk/btr/363-490 - essentially for IHT purposes accumulated income becomes capital after five years. (This is a gross simplification!)
Almost all Deeds of Trust will give the trustees powers to accumulate income as capital (usually by resolution rather than a full deed) and normally also allow the trustees to hold income as accumulated income on account and to distribute such income as if it were received in that year. The trustees should be maintaining proper accounts that show separate income and capital accounts and should have documented resolutions showing when they have distributed or capitalised income. If that is the case, then generally HMRC accept what is documented in the trust accounts and distributed income will be taxed as income on the recipient (with a tax credit) and distributed capital will be potentially chargeable to IHT. Though holding accumulated income for a long time or actually re-investing it but showing it as income is likely "on the edge".
If the trustees have not clearly and regularly dealt with the accumulated income, then the five year rule comes into play. At a 10 year anniversary, all accumulated income older than 5 years is treated as capital and so relevant property and so subject to the IHT charge. Note no concession is given for the fact that the income was not capital for the full 10 years - which it would be if the trustees were explicitly capitalising it.
In my mind there is still a grey area however, as the law technically only applies to IHT i.e. five year-old accumulated income is capital for a periodic charge. It does not explicitly state that income on account can no longer be distributed as income to a beneficiary. My conclusion, when I faced this as a trustee, was that provided you had clear accounts and written resolutions showing the trustees intention to accumulate rather than capitalise they were unlikely to challenge you and certainly not for periods of less than 5 years.
Re winding up a trust, a trust ends when all the capital and income is distributed. Usually, income can be distributed by resolution of the trustees, but often a capital appointment will require a Deed. It is not difficult to write a deed yourself - it just needs to say its a Deed and needs to be witnessed. The only complication is if there is IHT to pay and submitting final tax returns to HMRC. You also need to prepare final accounts for the beneficiaries.