From Wikipedia on Offa:
In about 789, or shortly before, Charlemagne proposed that his son Charles marry one of Offa's daughters, most likely Ælfflæd. Offa countered with a request that his son Ecgfrith should also marry Charlemagne's daughter Bertha: Charlemagne was outraged by the request, and broke off contact with Britain, forbidding English ships from landing in his ports. Alcuin's letters make it clear that by the end of 790 the dispute was still not resolved, and that Alcuin was hoping to be sent to help make peace. In the end diplomatic relations were restored, at least partly by the agency of Gervold, the abbot of St Wandrille.
Suppose this marriage pact goes through. Does this change anything with regard to European history? Does Charlemagne start paying attention to Britain more?