If purchased on the day either group save 1/3 off for 3+ travellers or my railcard which also gives 1/3 off for up to 4.
So either way really.
Southwest trains are currently running a promo deal where if purchased in advance it is £15 per person day return. This does not include underground.
The travel card cost £12 thus £27 each and the standard fare between zone 1 and 3 is £4.80 (£9.60 tot) giving £24.60
The cheapest on the day with travel card for 4 equates to £25.10 each using group or railcard.
A saving of 50p if you queue up and pay for your underground trip each way or for ease get on get off no hassle.