3 "Daughter, I deem that counsel ay most fit

Daughter > {Girl, maiden; also a term of affectionate address used by any older person to a girl or young woman} ay > ever, always fit > fitting, suitable

4 That of the time does due advantage take; 5 You see that good King Uther now does make

King Uther > (See HRB 8.23)

6 Strong war upon the paynim brethren, hight

paynim > pagan hight > named

7 Octa and Eosa, whom he lately broke

Octa and Eosa > (Octa was the son of Hengest (see 210.65:2).
Together with his kinsman Eosa he launched an attack on Uther
Pendragon in c. 475)
broke > overwhelmed, crushed

8 Beside Caer-verulam, in victorious fight,

Caer-verulam > Verulamium (i.e. St Albans, in Hertfordshire)