both legs but keep in mind that Eurostar and TGV´s need a reservation on top of the Railpass and you may be cheaper with advance fares :) 

There is a limited number of pass holder seats on the Eurostar between London and mainland Europe. On popular departures and during high season those sell out weeks, and sometimes months in advance.

The best place to see the availability of passholder seats and make reservations on the Eurostar is 

https://www.b-europe.com/EN/Booking/Pass#TravelWish

If you don't get any result, press "later trains" and eventually you will find the next available connection. 

If you have a mobile pass you need to generate a Pass Cover Number in order to make the reservation at b-europe. You do that here in the PCN generator:

https://community.eurail.com/news-and-announcements-39/pass-cover-number-generator-is-live-5653

Advance tickets are not flexible and the routing you are planning is fraught with expensive and infrequent trains. At a rough estimate a Manchester to Euston advance for a train early enough to meet  a lunchtime E* will cost at least £50 and the early trains are over £100 - all designated train and non-refundable if you cancel. Eurostar in summer are from £75 to £100+ and non refundable. Can be changed but with fees if within 7 days (plus any extra fare difference).

Paris to Barcelona TGV is min 130 euro to well over 200 euro with varying terms and conditions.

So for your itinerary you could be paying over £300 on advance tickets with little opportunity to recover any payments if you miss the specified train and excess fees for replacements (Man - Lon on the day is nearly £200). A 4 day pass for an adult is £227 and reservations would be 62 euros (about £50). So even if you have to change your plans the Global pass would allow you total flexibility (inc the prime train times) to travel when you want, not when there are cheap tickets.
Remember also that your spare day can also be used when you get back to the UK for a day trip as long as you travel Man - France in a single day.
 

I have managed to book a Eurostar train using this method. Thanks so much for the tips. Does this need to stay in that app and is there a way of connecting it to the Interrail app? Thanks in advance. 

There is no connection between interrail app and reservations, on trains that require them you need to have both res and pass with train added, on non-res trains you just need the pass.

Reservations are entirely separate from the app. You can't book them in the app and they won't be visible in the app.

It’s the seat reservations I have booked using the method described by Anna above. 
I show my pass on the Interrail app and the seat reservation on the app I booked my seats on? Is that right? Thanks 

Are there recommended apps to book seat reservations for trains through France to Barcelona and then onto Alicante? Thanks in advance for help. 

- GWR offers free seat reservations for UK trains (although they're optional anyway) or simply ask your local train station.

- Eurostar (30€) can be booked here : https://www.b-europe.com/EN/Booking/Pass#TravelWish (4€ booking fee per order)

or on interrail.eu (2€ per person per journey).

- TGV Paris - Barcelona (35€) should be booked on interrail.eu (2€ booking fee).

- For trains within Spain it's a bit more complicated you need to visit a Spanish train station (RENFE doesn't sell them online anymore). See advice here : 

Apps such as Trainline do not offer seat reservations. Always better to book with the companies themselves (but not possible with Eurostar, SNCF and RENFE... :/)

That’s very helpful, thanks.

Am I able to book my reservation for the TGV from Paris to Barcelona on the b-Europe site too? Like the Eurostar,  thanks  

No you need to use interrail.eu for that one (2€ booking fee) or you can call SNCF (no booking fee), they speak English. See :

https://travel.b-europe.com/Eurail-GE/en/booking-tgv#TravelWish This link only works for TGV rides within France

If you want to save a little bit of money, take a TGV to Perpignan (limited 10€, then 20€) and then take a regional train to Portbou and another one to Barcelona. It takes a bit longer but it’s way more scenic along the Mediterranean + you save money :)