Although not particularly prominent, there have been some statements and publications on this topic:
In this blog post by Jonathan Munro, the Deputy CEO of BBC News & Director of Journalism the debate is described as thus:
We’ve invited the seven biggest political parties in Great Britain to participate in our network election debates during the campaign.
it continues
I’m often asked how we gauge which political parties should be invited to our debate programmes during the election period. As per the BBC’s election guidelines and in line with Ofcom, we do this via past electoral performance, and parties’ sustained standing in political polling. We take into account the number of candidates a party might be fielding, either across the country or – in the case of the SNP and Plaid Cymru – in Scotland and Wales respectively. Our guidelines set out the broad levels of coverage each of the parties should expect to receive over the campaign period, across the UK and in Scotland, Wales, and Northern Ireland.
That post also links to the latest version of the election guidelines in question which includes a multi-section appendix on the coverage of parties. This in turn links to the OFCOM (the UK broadcasting regulator) advice from the local elections in May
- we place greater weight on the actual performance of a political party or an
independent candidate in elections over opinion poll data. This reflects the fact
that electoral performance is a measure of how voters have actually exercised
their democratic choice. This compares with the greater uncertainty associated
with support in opinion polls, which may not translate into actual votes or seats at
an election;
= in considering past electoral support, we take into account factors such as the
electoral performance of parties (including the numbers of elected candidates and
overall percentage of vote received) or independent candidates in the previous set
of corresponding elections over at least two electoral cycles;
- we also take into account performance in other relevant past elections being
contested at the same time, as well as performance in other recent past elections;
• we take into account the electoral performance of parties or independent
candidates over at least two electoral cycles when considering performance in any
given type of elections. However, we place less weight on the evidence of electoral
performance two or more electoral cycles ago given the historical nature of this
evidence;
- where relevant, we consider evidence in relation to electoral performance in the
different nations of the UK;
- while putting less weight on levels of current support as opposed to actual
performance, we put weight on evidence of current support that is objective and
measurable. One type of objective and measurable evidence of current support is
opinion poll data, where it is available. There may be other types of evidence of
current support but in considering such evidence we would take into account the
consistency and objectivity of each type of evidence; and
- our intention is always to undertake a balanced assessment having regard to the
totality of relevant evidence
As such, (and this is my own editorialising) it appears the Worker's Party of Great Britain has been penalised for a lack of history of consistent performance in elections (having only been founded in 2019) along with Galloway having won a by-election rather than showing any significant national support. On the other hand local news coverage in Rochdale and the surrounding areas should give due weight to Galloway and the Worker's Party as the incumbents.